Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
925 PM EDT Tuesday Mar 11 2014
a strong late-winter storm will impact the area on
Wednesday...followed by cold and blustery conditions with well-
below normal temperatures for Thursday. Temperatures will rebound
to near seasonal levels Friday into the weekend...ahead of a
frontal system pushing southeast from the Great Lakes. This front
may produce some rain or snow showers on Saturday.
Near term /until 6 am Wednesday morning/...
a nice mild evening is in progress as clouds continue to increase
from the west. Just minor tweaks to the near term temperatures and
I removed the mention of snow over the north later this evening
as it will still be some time before enough cold air slips south
to bring wintry precipitation into mention.
Overnight temperatures will dip down near freezing along the PA/New York
border...and remain in the 40s across the southern half of County Warning Area.
Moisture aloft will continue to thicken up tonight. Sub-tropical
high centered east of the Bahamas will supply the good srly flow
and real Gomex moisture to a strong system currently dropping southeast
from the High Plains into MO. The sub-1000mb low will deepen even
further as it rolls eastward along the weak boundary over the Ohio
Valley and northern Middle-Atlantic States. The low is prognosticated by all models
to cross the southern tier of PA Wednesday - at 12z it should be entering far
southwestern PA...and will deepen to a 980mb low by the time it scoots
east of kabe Wednesday evening.
Light precipitation will initially slide into the northwest tonight
/around 06z/...with 850mb temperatures above zero it should fall as
mainly rain...though some wet snowflakes may be mixed in. First
wave of moderate precipitation should be approaching County Warning Area western edge by
Short term /6 am Wednesday morning through Wednesday night/...
sub-1000mb low will continue to deepen as it rolls eastward along
weak boundary over the Ohio Valley and northern Middle-Atlantic States. The
low is prognosticated by all models to cross the southern tier of PA Wednesday - at
12z it should be entering far southwestern PA...and will deepen to a 980mb
low by the time it scoots east of kabe Wednesday evening. Very strong
and near- textbook kissing jet couplet will enhance the lift and
frontogenesis as it moves through the state. The warm air to the
south /highs should slip into the 60s in the lower susq/ and the
already- anomalously warm temperatures will keep the precipitation rain over all
but the far northern tier through at least middle afternoon Wednesday. Even the
far northern tier will probably have trouble accumulating early on what
does fall as snow...due to the middle- March sun and marginal temperature
profile. But eventually precipitation intensity will win out. Light east
wind could keep temperatures near freezing in the northern mts Wednesday
morning...so isolated pockets of freezing rain may exist in the
northern tier Wednesday...especially for a brief period as colder air quickly dives
southward. As the low deepens and passes to the east...the winds
will go northerly and increase quite rapidly. Sustained speeds in
the 20s and gusts near 40 miles per hour are expected as strong cold air advection drops
temperatures into the 20s in the north very quickly Wednesday afternoon...into central
sections between 21-00z and by 03z in the southeast.
Quantitative precipitation forecast in the 0.75-1.25 inch range across much of the north is most
likely going to be mostly rain...but the strong cold advection and
coupled jet in the afternoon will bring mesoscale snow band/S and
temperatures will then be conducive to allow accumulation. Thus...the
accumulations could reach 6 inches or more along the northern tier by
late Wednesday night...with colder air spilling further south late
afternoon/early evening producing minor accums in the mountains the cold
winds will help mins into the lowest single digits in the north. This
will also lead to low wind chills. Have transitioned winter
weather watch over to a warning for northwest counties with an advisory
for the remainder of northern tier extending down across the
Main threat in the south as low tracks across PA will be potential
for convection. Storm Prediction Center places southern PA in slight risk...and with
strong dynamics associated with this system...mentioned threat in
severe weather potential statement.
On the back end of the storm...quite cold air /as cold as -20c at
850mb/ gets driven into the region on a strong gusty northwest wind later
Wednesday night into Thursday...with by far the coldest day of the week in
store for Thursday. Wind chills should reach near advisory
criteria over the northern mountains and Laurel Highlands...range from 0 to
-10f over the Central Ridge/valley region and dip to around zero
in the lower susq valley.
Long term /Thursday through Tuesday/...
Thursday morning will be brutally cold and quite windy on the
backside of the departing storm system...with 25-40 miles per hour north-northwest wind
gusts supporting wind chill values as low as -20f in the
north/West Mountains and near zero in the southeast zones. Lingering snow
showers across north-central PA and the southwestern Poconos/Endless Mountains should
taper off by midday...with little to no addnl accumulation expected.
Gusty winds will gradually subside during the day Thursday with
daytime highs 25-30f below normal. This is very impressive for early
March with daily record low-maxes in jeopardy in several locations.
In addition...low temperatures Thursday morning across western PA will also be near
daily record. Suffice it to say...it may not be this cold again
(especially during the daylight hours) until next winter.
The good news is that the cold blast will be short-lived...with a
rapid moderation occurring on Friday. Breezy SW return flow in
advance of northern clipper surface low tracking across the upper Great Lakes
and downsloping should help to boost temperatures into the 40s north and
50s south. The models turn the low to the northeast up the St.
Lawrence Valley on day 5/Sat...dragging a trailing/weak cold front
across the northeast and middle-atl states. Model consensus indicates
some light precipitation is possible with chance probability of precipitation as the front shifts southeastward
across the County Warning Area.
Gefs/ec ensemble means show the large scale pattern reverting back
to a western ridge/eastern trough configuration into early next week. This
has been the favored/prevailing flow regime this winter...with a
cold polar vortex over eastern Canada. A strong surface high pushing
southeastward from the upper MS valley should direct colder air into the region
behind a reinforcing frontal passage on Sunday...with temperatures likely trending
lower by day 7/Mon. The non-NCEP oprnl models /12z CMC and European model (ecmwf)/
are more bullish on cyclogenesis in vicinity of the Gulf Stream just east
of the Carolinas...with the northwest edge of potnl precipitation shield possibly
reaching southern/southeastern PA on St. Patrick's day. Confidence is low given
the large spread between the GFS and ECMWF/CMC. Something to monitor
in the days to come.
Aviation /01z Wednesday through Sunday/...
VFR conditions will continue through the first half of the
Sat shows low to middle coulds streaming through with ceilings at
most taf sites. These will slowly lower as the night continues and
restrictions will arrive as light rain /and poss snow north/
around 06z in the northwest...and heavier area of precipitation arriving starting
around 12z Wednesday. Bulk of heavier precipitation will be between late
Wednesday morning and early Wednesday evening.
Widespread adverse conditions on Wednesday...with rain across much of
the area...snow across the far north...and some risk of strong
thunderstorms near the Maryland border. By afternoon...strong wind fields will
bring increasing northerly flow...llws...and gusty winds up to 40
miles per hour. These continue through the first half of Thursday as very cold air
stampedes into PA. With strong storm dynamics...concern exists for
a burst of heavy wet convective snow /and poss thunder/ near the
center of the low Wednesday/Wednesday night.
Have updated tafs with low level wind shear at all central and western taf sites
from 16z to around 22z.
Wednesday...widespread MVFR- IFR
restrictions in rain and snow. Strong north/northwest winds developing. Low level wind shear.
Thursday...strong/gusty north/northwest winds...gradually diminishing in the
afternoon. Fri-sun...no sig weather expected.
with warmer temperatures and midweek rainfall...expect moderate
rises on rivers and streams mainly for the southern half of the
County Warning Area. Still enough ice in spots to bring ice jams...though
significant decay has occurred at many locations. Any ice jams
will be isolated in nature. All persons should monitor water
levels closely for the rest of the week.
Winter Storm Warning from 8 am Wednesday to 8 am EDT Thursday
Winter Weather Advisory from 8 am Wednesday to 8 am EDT
Thursday for paz006-010-011-037.
Winter Weather Advisory from 3 PM Wednesday to 8 am EDT
Thursday for paz017-024-033.
near term...la corte/rxr