Area forecast discussion
National Weather Service Pittsburgh PA
212 PM EDT Wednesday may 22 2013
Synopsis...
upper level impulses brings an increase in showers and
thunderstorms through Thursday night...until a cold front crosses. Much
cooler weather is forecast this Holiday weekend under high
pressure.
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Near term /through Thursday/...
daytime heating continues to push our temperatures close to the
convective threshold. 16z mesoanalysis illustrates no cinh left
and equiv potential temperatures building. Still a fair amount of uncertainty
with the development of storms and how strong storms will be
later today and tonight. Latest thinking is storms will develop
along the Theta-E ridge axis which is building across WV and western
PA. Terrain induced storms are already underway over the central
mountains of WV. Farther west across Ohio...not sure how much development
will occur this afternoon until short wave trough passes around 0z.
Likely probability of precipitation still are on track for this evening for the entire
area. If the latest hrrr is correct we could see some localized
high water potential over northern WV with training of storms.
After midnight things should quiet down as llvls stabilize and we
await main trough axis to traverse from the west. NAM and GFS agree
on a period of dry or isolated activity tmrw from late morning through
middle afternoon before upper level impulses cross. Do not foresee a
severe weather threat Thursday due to lack of heating as an
abundance of clouds will be presence keeping thermodynamics low.
Greatest challenge is if NAM scenario of a batch of thunderstorms riding
NE from WV into southwestern PA around daybreak materializes. For
now...held off in favor of continuity per low confidence on this
outcome. 98
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Short term /Thursday night through Saturday/...
a rapid transition to much colder conditions takes place during
this period as the consensus of the more reliable global model
systems suggesting the middle-level system tracking through the
region should close off as it moves over Pittsburgh and slowly
oozes southeastward. The closed solutions favored by the more
reliable models end up trending toward the slower end of the
guidance pack with this system...which makes sense given the
higher long wave amplitude. As such...the systems departure was
slowed in the forecast which resulted in increasing probability of precipitation into
Friday.
While its departure is slower...much drier air does work into the
area by Friday night with the consensus offering extremely lean
layer relative humidity profiles and weakening northwest cold advection. This
sets the stage for stellar radiational cooling going into Saturday
morning. Gridded guidance suggests widespread 30s across the
area...which with 850 mb temperatures falling toward -2c is not too
shocking. Frost was added to the grid to accommodate lows
generally in the middle 30s across vast tracts of the County Warning Area...and
frost/freeze headlines seem possible at this point. These
temperatures at 850 mb fail to moderate during the day Saturday
much...so temperatures will likely only wander for highs from the
upper 50s Friday toward the lower 60s Saturday. Fries
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Long term /Saturday night through Wednesday/...
similar conditions persist through the rest of the weekend and
even possibly into Monday morning with very dry conditions and
light winds favoring Superior radiational cooling each night. As
such...lows continue to look to head toward the 30s each
night...with a few locations inevitably dropping below freezing.
Once again...frost was added to the forecast into the
overnight/early mornings this weekend...and frost/freeze headlines
again seem quite possible.
Otherwise...very gradual moderation of low level temperatures will
be likely as the closed system departs eastward but dry cyclonic
northwesterly flow aloft remains the large controlling factor for
our weather. This is thanks in part to the fact that the model
consensus keeps the large scale ridge axis largely to our west
through the middle of next week. As a result...conditions should
remain generally dry with very slow moderation of temperatures
through the week. Fries
&&
Aviation /18z Wednesday through Monday/...
low to medium confidence forecast on shower and thunderstorm timing.
VFR weather will prevail much of the forecast period /at least into
the overnight hours/...the challenge is timing on any rain showers or thunderstorms and rain
impacting the terminals. Maintained thunderstorms in the vicinity for late afternoon and
evening before transitioning to VFR rain showers until midnight. Clouds
will develop in the predawn hours...latest guidance suggest ceilings
in the 015-025 which is higher than earlier runs and in line with
ongoing forecast. Stratocu will lift by middle morning into a low end
VFR deck. For places that experience rainfall...patchy fog could
develop during the predawn hours. 98
Outlook.../Thursday through Sunday/...
the probability of widespread IFR weather is low.
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Pbz watches/warnings/advisories...
Maryland...none.
Ohio...none.
PA...none.
WV...none.
&&
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