Area forecast discussion
National Weather Service State College PA
728 am EDT Thursday may 23 2013
Synopsis...
an amplifying upper-level trough and its associated cold fronts
will cross the northern Middle-Atlantic States over the next 48
hours. Canadian high pressure will build southeast from the Great
Lakes through the Memorial Day weekend. The overall pattern will
be unsettled through Friday with improving conditions expected
during the weekend. Temperatures will trend colder over the next
few days...before likely moderating through the early to middle
part of next week.
&&
Near term /through tonight/...
WV satellite imagery shows two shortwave troughs rotating over
Hudson/James Bay and WI/Michigan early this morning. This energy will
phase and dig into the western Appalachians by the end of the period. The
00z guidance has come into better agreement with the details of the
heat fall evolution and are well clustered through f36. At the surface...a
cold front draped across southern Ontario/Lake Huron into central lower Michigan will
push southeastward across the lower lakes this afternoon before crossing through the region
late tonight. A pre-frontal trough will shift eastward across central and eastern
portions of the County Warning Area this afternoon.
Mesoscale models show a gradual increase in shower activity through the
morning hours...coinciding with approach of large scale forcing/heat
falls. The main concern in the near term is again potential for
severe convection with the focus shifting from west-central to east-central
PA. Storm Prediction Center has upgraded the eastern 1/3 of the County Warning Area to slight risk. While
instability/destabilization potential may be somewhat limited by
persistent band of multi-layered clouds streaming northward within moist
south-southwesterly flow aloft...breaks will likely occur and result in modest ml
cape values. This should be sufficient for storm initiation in the
warm sector /perhaps as early as 16z with peak intensity reached
"early" in the 17-21z timeframe per hrrr/ with the main thunderstorm
development along pre-frontal trough located in vicinity of the susq valley.
Belt of stronger middle-upper level flow/30-40kts of effective shear
along the eastern periphery of the approaching upper trough will be
available for gradual storm intensification/organization of
multicells capable of producing damaging winds. Any severe threat should
diminish by the early evening.
A potential secondary hazard may be locally heavy rainfall. A
Reservoir of above normal precipitable water feeding northward from the
western Atlantic along with decent blyr convergence/upper level divergence
couplet should support moderate rains over the eastern half of the County Warning Area with
amts between 0.50-1.00 inch. Localized amts approaching 2 inches are
not out of the question with the greatest risk along and east of
the susq river.
Cooler...more stable air will begin to work into the area tonight in
the wake of the cold front. The precipitation process becomes more
frontogenetically driven as models depict a deformation axis pivoting
across north-central PA late tonight into early Friday. Quantitative precipitation forecast amts will be on the
light side /0.10-0.25 inch/ in the 00-12z period. The sensible weather
pattern will transition from recent Summer-like heat to cool/damp
and cloudy conds with an increasing north/northwest wind.
&&
Short term /Friday through Friday night/...
model data all indicating digging trough will cut off over western PA
on Friday. Strong lg scale forcing ahead of this feature...as depicted
by model 500-300mb q-vector divergence...should support a good chance
of rain showers in the cool air behind the surface cold front.
Have placed the highest probability of precipitation /nr 80 percent/ across the north central
mountains based on latest gefs output and anticipation of orographic
enhancement along The Spine of the Appalachians. The arrival of
much drier air wrapping into the west side of developing cut off
will likely shut off any showers early in the day across Warren
Colorado.
A tightening pressure gradient between surface high over the lakes and
developing coastal wave will result in a blustery day for late
may. BUFKIT soundings indicate northwest winds will be gusting between
25-30kts during Friday afternoon. Temperatures in the 50s...combined with these
gusty winds...will likely result in apparent temperatures in the 40s
during the afternoon across the alleghenies.
Main concern Friday night will be the potential of a freeze across
the Allegheny plateau. At this time...aforementioned pressure gradient appears
likely to keep the winds up Friday night...so widespread frost seems
unlikely. Hard to believe there will be a widespread freeze this
late in the season without a calm wind...so have leaned on the
warm side of MOS /closer to ec MOS/ and kept temperatures just above freezing
for most of the West Mountains Friday night.
Model trend has been to slow eastward progression of upper low...so
have kept mention of rain showers in the forecast through Friday night across the
eastern zones...and tapered them off later across the central
counties.
&&
Long term /Saturday through Wednesday/...
eastern PA will remain on western periphery of upper low Sat/Sat
night. So...have increased the cloud forecast a bit over the eastern
counties and can/T completely rule out a stray rain showers over Sullivan
or Schuylkill Colorado. Elsewhere...confidence in a mostly sunny Saturday is
fairly high...as all model data pulls low precipitable water air southward across
central/western PA on Sat. However...despite strong may
sunshine...model 800 mb temperatures well blw normal should cap readings in the
l/M 60s across much of the area...with highs in the u50s likely
across the high terrain. Deep mixing should draw some gusty winds
to the surface again on Saturday. 800mb wind fields suggest much of
the area will see gusts between 25-30kts by afternoon.
Bulk of medium range guidance continues to indicate a high degree of
confidence in dry weather holding through Memorial Day...along with a
gradual warming trend...as upper low over new eng slowly lifts away
from the area.
By the middle of next week...a building upper level ridge is
advertised by the medium range guidance over the miss valley. Across the
east...surface high will pass off the coast...resulting in a warmer SW
flow. Some model diffs with respect to how fast the low level moisture
returns. However...have gradually introduced a chance of diurnal
convection in the Tuesday/Wednesday time frame.
&&
Aviation /12z Thursday through Monday/...
5/23/12z...some restrictions now being observed across the airspace
mainly due to low ceilings. However expect VFR to resume within a few
hours. Thunderstorm impacts will be the main focus from late morning into
the afternoon.
Lg scale forcing associated with approaching upper-level trough and
surface cold front...impinging on Reservoir of above normal moisture
streaming northward into the region...will result in numerous showers and
thunderstorms from late morning through the afternoon. Some strong to locally severe
thunderstorms are possible across the eastern 1/3 of the airspace...mainly in
zny sector. Expect mainly VFR conds with occasional MVFR restrictions.
The upper trough will amplify as short wave digs across Virginia on Friday and
may even form a cut-off/closed low. This will result in a low
ceilings and -ra with a breezy north-northwest wind. There is some uncertainty
with how fast this system will exit the area heading into the
weekend. However the worst flying conds should be on Friday with
gradual improvement as Canadian high pressure builds southeastward from the
Great Lakes.
Outlook...
Friday...low ceilings with area of light rain. Gusty north-northwest winds.
Sat-Mon...VFR. No sig weather expected.
&&
Ctp watches/warnings/advisories...
none.
&&
$$
Synopsis...steinbugl
near term...steinbugl
short term...Fitzgerald
long term...Fitzgerald
aviation...steinbugl
