Download some-synoptic aspects of a change in weather regime

E'ERRVARY
MONTHLY WEATHER REVIEW
1950
31
SOME-SYNOPTIC ASPECTS OF A CHANGE INWEATHER REGIME
DURING FEBRUARY 1950
HERMANN B. WOBUSANDLEWIS
C. NORTON
WBAN Analysis Center, U. S. Weather Bureau
Washington. D. C.
INTRODUCTION
Beginning near the end of the first week and ending
during the t.hird week of February 1950, theweather
regime overtheUnitedStatesunderwentacomplete
change from the abnormal conditions which had persisted
during Januaryandtheearlypart
of February.The
change was marked by a moderation of extreme cold in
the northwesternportion of thecountry(andwestern
Canada) and a return from unseasonable warmth to more
normal conditions in tJhe East. T h e trarlsit'ion period was
accompanied by a series of stormsmovingacrossthe
United Stat,esandCanada, one of which producedthc
first heavy and last,ing snow cover of t'heseasonin the
northeastern part, of the United St'atcs. The procc'sses by
which achange-overfrom
one regimc to anothertakes
place are not fully understood, but' in the present discussion somc synoptic aspects of the chungr-over arc traccd,
together x-it.11 certain associated physical and dynamical
factors, wit~hparticular emphasis on inertia cffcets widcnt
FIGCREl.-iYormal
sea level isobarsat. 3-mh. intervals (solid lines) and700-mh. contours
at 200-footintervals (dashed lines litbclprl in hundredsof Icet) for themont,h of February.
in observed upperwindpat.terns
[1,2]. An attempt is
made to point out features related to short-term forecastingproblemsratherthanto
explain the causes of the
change-over.
GENERALSYNOPTICFEATURES
Thc anomalousfeatures of the circulation during the
early part of February may be seen by comparing figures
1 and 2. Figure 1 (based on [3,4]) shows the normal sealevel pressure and normal 700-mb. pattern for February,
and figure 2 the corresponding average conditions for the
first 5 days of February 1950. Figure 2 shows that the
cold air flow, which is normallysouthward across the
HudsonBayareaintonortheasternUnitedStates
was
displaced far to the westward, resulting
in cold weat'her
in western Canada and northwestern United States and
mild conditions over the United States east
of the Rockies.
Other important anomalies in figure 2 include a cold surface High ovcr the Great Basin
region, and a large surface High over the Aleutian Islands, the latter
being of
FIGVILE
2.-Mcan sea level isobars at 5-mb. intervals (solid lines) and 7W-mb. contours
at 200-foot intervals (dashed lines labeled in hundreds of feet) for the period February
1-5, 1950.
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MONTHLY WESTHER REVIEW
FERRUARY
1950
FIGURE
3.-Mean sea level isobars at 5-mb. intervals (solid lines) and 700-mb. contours
at 260-foot intervals (dashed lines labeled in hundreds of feet) for the period February
8-12, 1950.
FICTXE4.--North Bmcrican surfacc map for 1830 GILIT, Fcbruary 8, 1950. Shading
indicntcs arcits of actire prccipitation.
particular importance in separating normally
low pressure in the Aleutian region into two Low cells, one near
Japan and the other in theGulf of Alaska.
Figure 3 shows the mean sea-level pressure and mean
height of the 700-mb.surface for February 8-12, 1950.
There was still no appreciable flow of cold air from the
Hudson Bay area into northeastern United States; inst'ead,
cold air wasmovingaroundaHighwhich
had formed
in the Hudson Bayregion. T h e High near Salt Lake City
had moved southeastward since the early part
of February,
and a new Pacific air mass was moving into thc region.
Temperatures hadmoderatedin the Pacificnorthwest
\vhcrc
a new and weaker surface High was forming.
A very significant change had occurred with rcspect, to
the, surfaceHigh that waspreviouslyintheAleutian
region. A break-off of the Low in t'hc vicinity of Japan
had moved northward and eastward t'hrough t'llc Bwing
Sea, separating the Aleutian High from its connection to
theSiberiananticyclone.TheHigh
had given may toward the sout'heast and was decreasing rapidly in intensity. Cold air o n the east side of this High was feeding
partly into the Low in the Gulf of Alaska and partly into
an east'ward-moving Low at about lat'itude 85' N. On
February 8 (fig. 4) a portion of this cold air, its ternperature modified by thc ocean trajectory, moved into British
Columbia,Washington,andOregon,andsubsequently
movedeastward across thecountryaccompaniedbya
Low which passed over the Great Lakesregion and skirted
FICVRE5,"North
the Hudson Bay High.
Bnlcrican surldce map for 1830 G M T , Fcbruary 10, 19.50. .Shading
indicates a r e a of aotive prcripitntion.
F I ( , ' I < R.--SoIth
E
American surface map for 1830 GAIT, February 16, 1950. Shading
indicates a r ( w of acatirc yrrcipitntion.
FIGURE7.-Xorth
Americansurface map for 1830 OMT, February 14, 1950. Shxling
indicates areas of active prccipitation.
.Is t'llis Low movedeastwardfrom
t'heLakes region
(fig. 5 ) colder air fromtheHudson
Bay High,moving
sout~ll\vard behindthe Low, flooded the Northern Ylains,
IA~C
region,
S
and Sorthcast~crn States. This cold air may
lx: seen on the surface weather map for 1830 GLIT, February 12 (fig. 6) after' t h e Low had moved into the dt'lantic;
it is north of t,he frontextendingfromConnecticutto
Indiana ant1 ovcr t h e area thence westward to the Rockies. Howcvt?r, t~ompcratureswere notunseasonably low.
Llcanwhile,onFebruary
10 (fig. 5 ) , anothersurge of
cold maritirncair passed i n l a r ~ lalong the Pacific coast
bet,wccn Juneau ant1 southern California. This deep mass
of cold air had moved east'wmd and southward into the
Texas and Oklahoma Panllandlcs and western Kansas by
1830 G51T of the 12t,l1(fig.6) a t which time low pressure
over Texas was associated with interaction between this
cold maritirnc air and the colder air that had previously
moved southwardfrom t h e Hudson RayHigh. Accompanying t.his c>ondition,as shown on t h e chart for the12t11,
was thc only strong flow of t'ropical air northward from
the Gulf of Mexico into t h e lower hlississippi Valley during
t h e month of February. Also, surface pressure ovcr Texas
was byfarthe
lowest duringthemonth
in thatarea.
This lorn pressurc intensified and moved northeastward.
On t,hc afternoon of the 14th (fig. 7) it was a well-developed
cycloniccirculationcenteredovernorthernOhio.
Also,
on the afternoon of t'he 14t8h, conditions werefavorable
for the formation of a secondary Low in the Chesapeake
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MONTHLY WEATHER REVIEW
FEBRCARY
1950
A sharpening of the trough is accompanied by some conBay region. Aloft,a
low-pressure trough, whichwas
(stir parcels
necessarily associatedwiththesurfaceLowover
Ohio, version of kinetic energy to potential energy
was moving eastward,accompanied by falling pressure move outwardacross the isobars or contours towardhigher
along the Atlantic coast. At the same time cold maritime pressure) and since thiseffect takes place in the middleand
air was moving from t8he east, into New Jersey and Dela- upper troposphere, it should result in lifting (and cooling)
of air from below and "drawing down" of air from above
ware in such a way as to be
deflected southward by the
region of resultinghorizontal
divergence. Some
Appalachian Chain, while to the southward along and off ina
the coast warm tropical maritime air was moving north- understanding of the kinematical, dynamical, and thermal
results of this process aids in anticipating the changes of
ward.Thenetresult
of these effects was to producea
secondary Low along the Atlantic coast which on the 15th pressure pattern, surface and aloft, that,will result from a
surge or series of surges. It explains, not the large scale
waswell
developed and whichproducedtheextensive
patterns of the upper troposphere, but rather some of the
snow cove,r over the Northeastern States. The secondary
day to day changes in the meandering stream
of upper
Low was off Nova Scotia on the 16th (fig. 8).
The extensive and rather heavy precipitation associatedwesterlies.
I n figures 9-16, trough lines or surges are indicated by
with both the mainLow and the secondarywas a result of
the strong northward flow and consequent liftingof moist symbols A, B, C, etc., with a subscript to indicate day of
the month; for example, A, refers to trough A on the 8th
tropical maritime air over the deep cold air mass.
on the gt'h. These lines
are
Another outbreak of cold air, which on the 16th (fig. 8) and A9 to the same trough
referred
to
alternately
as
troughs
or
surges,
depending
on
was approaching central Canada, movedacross the northwhrther
or
not
a
surge
is
associated
with
the
trough
line.
eastern part of the United States on the 18th and 19th,
In figure 9, the chart for February 8, it, may be noted
preserving the snow cover in that region and definitely
that
along trough As the reported winds south of latitude
ending the regime of unseasonably mild winter weather in
45'
N.
and behind the trough, in Kevada,
were weaker
that area.
thanthewindsahead
of thet'rough. In Montana the
windswere stronger than t'hose reported in Minnesota.
FEATURES OF THE UPPER TROPOSPHERE
Thermal advection (warm advection indicating a rise and
cold advection a fall of pressure) was an indication that
The preceding discussion outlines briefly some of the the ridge along the ninet'ieth meridian would move rapidly
main synoptic features of the change in regime. Extreme eastward,andthat'pressuresaloftalongthe
ridge and
cold in the Northwestern States moderated by thc end of immediatelywestwardwouldfall.On
t'he 9 t h (fig. 10)
the firstweek in February but the
final phase of the change the trough A, was slightly sharper in the
region of the
in regime, ma,rked by a change to colder and more normal wcsttlrn Great, 1,akes; theairthenpassingSaultSte.
weather in the easternpart of t'he country, took place only hlaric had moved through the trough and L'oversllot'' it,
afteranintervening
series of synoptic cvcnts. These hcing first' drc3eIcraterl and later cIrflrcted to the Ieft upon
synoptic events, discussed so far mainly with respect to
entcring t'he gradient east of the trough line. Meanwhile,
surface
conditions,
were
accompanied
by
important
t'he southernportion of thetroughhadadvanced
only
change,s of circulation in the upper troposphere, some as- sIo~vly, t'he smalltroughinsout'hwest'ernUtah
on the
pects of which we examine in further detailin this section. 8th havingbeen flattened into broad curves in the southern
Upper air conditions are shown in
figures 9-16 inclusive, Ylains area by the 9th. By the 10th
(fig. 11) this trough
the daily 500-mb. charts for 0300 GRilT, February 8-15. was virtuallyspent.There
wasstilla
pattern for cold
On these charts a series of surges are followed, and their advection, suggesting somefall of pressuretowardthe
implied effect on the changing configuration of uppcxr air east. of trough AI,,, but the gradient ahead of the trough
pressure and flow patterns is discussed. For this purposr was sufficient to deflect northeastwardand
accelerate
a surge is defined as the leading edge, marked by a wind slower moving air advancing through tmhetrough line AIO,
shift or trough, of an areaof strong wind. A good example wit'h consequentconvergence t'hattendedto
offsetany
of a surge is one which approached the coast of California tendencytoward
rising pressurealoftinthe
middle
and Oregon on February 10 (fig. 11); on the 1l t h (fig. 12) Atlantic region.
a secondary surge movedinto centralOregon a.nd northern
Looking again at the chart for the 8th (fig. 9) wefind
California. A surge may or may not be associated with a trough B, off the Pacific coa,st. Its sharpnesswas not
front.
well defined because of scarcity of data in the area, but
An attempt is made to follow in this series of 500-mb. the wind report fromt<hestationary shipa t 50' IS., 145' W.
flow behind the
charts an idea that has
been utilized to some extent by the indicates that there was much stronger
trough t,han is suggested by the existing gradient
along
WBAN Analysis Center in forecasting cha.nges in upper
(fig. 10) there
level configuration. This idea is, in substance, that a surge theBritishColumbiacoast.Onthe9th
were two minor surges, B, and B',+ The double structure
of morerapidlymovingair
parcels, on overtakingthe
troughline,contributesaportion
of itskinetic energy here was evident once the system was over land, and it
toward sharpening the trough line,
or conversely, R S the may have existed over the ocean, undet'ected, or it may
surge deteriorates, the sharpness
of the trough deteriorates. have resulted from topographic effect$.
FEBBUARY 1950
MONTHLY WEATHER REVIEW
35
For ea,ch of these troughs, B, and BT9,the gradient and kinetic energy was being spent partially in deepening the
the wind upstream from the trough exceeded t h e gradient trough. The winds reported by an Air Force reconnaissanceflightnorthwestn-ardfromthecent'ralCalifornia
wind in the t,rough. Also, theairmovingthroughthe
trough advanced into a region of weaker actual gradient, coast (fig. 11) were sufficientlyst'rongfor air parcels to
and as a result tended t,o follow a path somewhat south movc far t'oward thesoutllcast, in spite of the gradient.
Gradient in the area into which these winds were moving
of the contour lines existing on the chart for the 9th.
Figure 11, for February 10, shows that troughs B,, and was sufficiently weak, especially off the California coast,
BTl0were stillinevidence, but Bl10wasweak. Blo was topermit a longcross-gradient' trajectory before they
becoming thedominanttroughas
could be decelerated sufficiently to recurve toward
lower
A,, weakened.The
gradient winds overMont~anaandWyoming
exceeded pressure.Consideringthe
succession of parcels wllich
those inthe region of troughs B and B'. Also their were advancingtowardthetrough,eachparcel
passing
direction led toward an areaof weaker gradient in Illinois through the trough would decelerate, contributing to the
and Wisconsin. This called for a drop in pressure in thc deepeningandadvance
of t'he trough,and
forminga
Lakes region as tlle air entering the region of weak gradi- pressure pattern into which each succeeding parcel would
ent crossed contours toward the south, or away from the
advancealittlefarthersouthward
before it spentits
Lakes. The result may be seen on the chart for the 11th
kinct,ic energy in deepening the trough.
Thisrcsultingsouthwardmovement
of mass, uneom(fig. 12) where only a single trough, designat'cd as RI1, is
now in evidence. The same reasoning applies to the New pensat,ed by similarsouthwardmovements
of massin
England area for the 12th.
some area far to the north (in this
case Alaska and northern
On the9th (fig. 10) there wasnnot'hcr trough in the Canada) required a fall of pressure aloft along the Pacific
Pacific, Cg, extending southward from the Low
that had coastal arms, as was verified on February 11 (fig. 12).
persisted in the Gulf of Alaska. TTind a t 50' N., 145' W.
Onthe10th
(fig. 11) therewas a possiblesecondary
had decreased since the previous day, and in the meantimetroughforming at Dlo. Its existence was notcertain at
warmer air had moved into that area accompanied by a this stage, but was indicated in reconnaissance wind remarked rise in theheight of the 500-mb.surface. The port's near the California coast. By the 11th (fig. 12) the
wind a t 50° N., 145' W. (fig. 10) was about 15 knots posit,ionfor this trough and its existenceseemed better
stronger than the gradient wind for the contour spacing
established .
and curvat'ure
at'
the
corresponding
point
along
the
Meanwhileanothertrough, E l l , wasbecomingestabtrough. This means that such air parcels as were repre- lished in New Mexico. Winds a t Albuquerque and Tucson
sented by this wind decelerated upon passing through the bear out the idea that winds approaching trough C moved
trough, thenrecurvedsharplytowardthenorthcast,
far to the south of the contour pattern until they
were
carrying warm temperatures toward the British Columbia decelerated sufficiently for the weak gradient in trough C
and southern Alaska coastal region, resulting in a
rise of recurve
to
them
northward.
When
recurved,
they
upper level pressureinthat'area.By
t'he 10th (fig. 11) crossed contours toward lower pressure, being accelerated
untilagain
of sufficient speed to recurve to theright
this had taken place.
Meanwhile the Low inthe Gulf of Alaskahad been because of tlleCoriolisforce.
The leading edge of air
deflected westward, the upper level pressures to t'he west moving in this schemeseems then to have beenalong
and southwest of the Low falling as a deep cold air mass E ll. Again, as with the system B, there may have been
moved southward through western Alaska and then east- topographic effects which contributed to the observed
ward around the Low. This movement of cold air south- effect.
ward, theneastward, becomingparallel toasustained
Trough E persisted and advancedsince the winds behind
flow of warm air immediately southward and moving
in it were in excess of the gradient winds in the trough, and
the same direction, represented a buildup of solenoidal or pressures in the trough fell because the speed of the wind
potential energy, and also appeared as a strengt'hening of in New Mexico and Arizonacarried air well across the
the gradient for westerly winds in the vicinity of latitude gradient in advance of the trough. Meanwhile trough
C
45' N Thisstrengt'hening of gradient deflected north- persistedbecause
thegradient wind withinthetrough
ward (or toward lower pressure) any air that was entering was slight,ly weaker than the wind reported on the Calithe area a t this level. Then cross gradient flow accel- fornia coast.
erated ituntilitsspeed,andconsequent,lythe
Coriolis
Meanwhile, surgeD 11 (fig. 12) was becoming a dominant
force, was sufficient to recurve it toward the south. ,4s it feature. While C 11 moved eastward, D 11 was "crashing"
entered the area eastof the strongest gradient, the
Coriolis southward, becoming sharper and deeper, and tending to
force deflected it farther southward toward higher
pres- merge with Cll. As a result, therewas further deepening in
sure, especially as it passed into and through the trough
the general trough somewhat ahead of trough D, as may
Clo. This resulted in deceleration of the air parcels.
beseen by comparing figures 12 and 13. On figure 13,
By the time of t'hr chart for the10th (fig. 11) t,his decel- the trough line C 12 was discontinued north of the Low in
eration was taking place near the California coast, where Arizona and trough line D 12 extended into the trough in
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MONTHLY WEhTHElZ REVIETV
FEBRUARY
1950
MONTHLY WEATHER REVIEW
37
38
MONTHLY WEATHER REVIEW
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1950
FEBRUARY1950
880442-50-2
39
15. Figure 16 representsthcupper
the northin order toemphasize the homogeneous character chartforFebruary
air
pattern
approximately
9 hours after the surface conof the flow approaching this line, in contrast to the nondit,ions
shown
in
figure
7
for
February 14.
homogeneous patterninadvance
of trough D
From
considerationssimilar
tothosealready
discussed,still
The 500-mb. pattern of February 12 shown in figure 1 3
further deepening was to be expected in the general trough immediately preceded the development
of surface storm
ahead of Dl, (fig. 13) in thesouthwesternpart
of the conditions over Texas as shown in figure 6. The developcount'ry. Farther north, particularly in Canada, winds to ment of t,he surface storm was a necessary result
of the
the west of D were weakening,indicatingnofurther
lowering of upper-level pressurc over the southern Plateau
deepening of the trough in that area. The result
is vcri- areaandtheadvance
of thoupper-levellow-pressure
fied on the chart for the 13t,h (fig. 14).
area eastward. The
fall inpressure at the surfacetook
On the13th
(fig. 14) thegradientwastemporarily
place somewhat to the eastward of the upper-level fall of
stronger on the west side of the Low which was then in pressure because of northward low-level advection of warm
New Mexico. Thisinturnacceleratedtheairmoving
air in advance of the upper-level trough, requiring from
into that region, as seen a t Phoenix andTUCSOK~,
but ot'l1r.r- elementaryhydrostaticrelationshipsthatthe
surface
wise the speed of winds upstream from the main
t'rougll Low be cast of the upper-level Low. Further intensificahad by then diminished. This chart
(fig. 14) is the first
tion of t,he surface Low was aided by the already
wellof the series in which the speed of the air leaving the Uniteddeveloped
upper-level
trough,
and
its
northeastward
States in the east approached or equaled that entering t'he movemmt was in response to the combined effect of castcountryfromthewest.Exceptforstrongwinds
in \\rad movement of the upper-level trough and the northArizona, and those entering the area
of very weak gradient wardadvection of warm air a t low levels ahead of the
ahead of t,rough E13and ahead of trough D13in eastern
upper-level trough.
Wyoming and eastern Montana, the winds
were mostly
REFERENCES
in fair gradient balance with the gradients they
were to
enter.
1. J. R. Fulks, Some Aspects of Non-gradient Wind Flow,
Divergence occurred in Iowa because of the strongwind
paper presented at American lCleteorologica1 Society
in Missouri,and in Nebraska and South Dakota
in advancr
meeting, New York, January1946, revised May 1946
of the weakening shear line D13. Thestrong wind in
(unpublished).
Arizona strengthened the gradient in the southeast part
2
.
Hermann
B. Wobus,ASystematicMethod
of Wind
of Texas. The light winds in Colorado and New hlcxico
Forecasting
a
t
500
mb.,
paper
presented
a
t
American
were deflected toward the lower pressures of the Low, and
llctcorologicalSocietymeeting,Washington,
April
contributed convergence in the southwest limbof the Low.
I948
(unpublished).
By the 14th(fig. 15) these effects were well toward accomplishment. The only winds then nlarkedly nongradicnt i n :3. U . S. Weat'ller Bureau,Normal Weather Alaps, Northern
Hemisphere Sea Level, Washington, D. C., April 1946.
speed were the lightwinds betwcen the Low and thc decaying shear line DI4. These were to be doflected toward the 4. Joint
lleteorological
Committee,
Normal
Weather
M a p s , ATorthern HemisphereUpperLevel,
Washingsouthern portion of the Low and t,o aid in its northcastwardacceleration.
T h e result is shown in figure 16, thc
ton, D. C., October 1944.