021
FXUS61 KCTP 041915
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
315 PM EDT Thu Jul 4 2024

.SYNOPSIS...
Warm and humid conditions will prevail through Saturday with
daily chances for showers and thunderstorms. A few storms could
produce strong winds and heavy rainfall this afternoon and
evening, especially in southern PA. Low pressure tracking north
of Pennsylvania will drag a cold front east of the state on
Saturday, ushering in drier and more comfortable conditions for
Sunday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
A weak, slow-moving and decaying cold front extending from the
Finger Lakes to the NW Mtns of PA will gradually drift south
through this afternoon. HREF indicates that MU CAPE will climb
to between 1000-1500 J/KG across the southern half of PA though
the latest RAP shows pockets of 1800-2300 J/KG sfc-based CAPE
values coincident with areas of more significant clearing late
this morning.

Vertical shear remains quite weak and some layered mid level
clouds will be increasing from the west and overspreading the
NW Mtns and Laurel Highlands this afternoon.

The Central and Lower Susq Valley appears to destabilize the
best today and stand the best chance of SHRA/TSRA, though the
eastward progression of a MCV from the Ohio Valley will also
spread late afternoon/evening showers/embedded TSRA across the
Western Mtns.

Vertical shear is rather meager, so pulse storms or some multi-
cell clusters will be the primary storm mode/threat this
afternoon and evening.

Still, the combination of instability and marginal shear yields
decent updraft helicity values across the southern tier of the
state, where isolated late day severe weather appears possible.
The presence of +2-3SD pwats could yield torrential downpours in
some parts of Southern PA Thursday afternoon. The 00Z HREF
indicates localized amounts to 3 inches are possible across the
southern tier, but dry ground in this part of the state will
limit the flash flood threat. Highs this afternoon will
generally be in the 80s with a few spots potentially reaching 90
degrees in the southeast before convection arrives.

Passage of the shortwave and nocturnal cooling should result in
dwindling convection Thursday night. Slightly cooler air may
work into the N Mtns, as the front sags into the central part of
the state. Otherwise, expect a very warm and muggy night with
min temps in the upper 60s to low 70s. Patchy fog appears
likely, especially in locations that receive rainfall today.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Latest high resolution guidance indicates a slug of heavy
moderate showers could be present across northern PA on Friday
morning, helping set the stage for more rain later in the day. The
stalled front over Central PA is progged to return north as a
warm front Friday PM ahead of low pressure lifting across the
Great Lakes. Falling heights and surging pwats ahead of an
upstream shortwave and associated surface low lifting across the
Great Lakes should bring a round of showers/tsra in the Friday
PM to Saturday AM timeframe. Model shear/instability profiles
indicate strong to severe tsra can`t be ruled out Fri PM.

Current guidance suggests the mid level shortwave and bulk of
the large scale forcing will pass north of PA, leading to
unimpressive mean qpf in the ensemble plumes of 0.25 to 0.50
inches Fri PM to Saturday. However, locally heavier amounts will
accompany stronger convection. The Weather Prediction Center
has placed all of Pennsylvania in a Marginal Risk of Excessive
Rainfall for 8am Friday through 8am Saturday to account for
the multiple rounds of showers and potential for rainfall
amounts approaching 2 inches in a few spots. Flash flood
guidance across most of the region remains quite high (3" or
more in 3 hours), so most of this rainfall should be beneficial.

It is also worth noting that heat index values will approach
or briefly touch 100F on Friday afternoon in southeast PA.
Dewpoints in the low to mid 70s combined with high temperatures
approaching 90 will result in a very hot and humid afternoon.
With scattered convection in the forecast, there is not yet
enough confidence to warrant a heat advisory but one may need to
be considered if temps/dewpoints trend higher. Overnight lows
into Saturday will also be quite mild, ranging from the upper
60s in northwest PA to the middle 70s in the Lower Susquehanna
Valley.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
The aforementioned cold front is expected to mostly clear the
region by Saturday morning, but some lingering showers across
eastern Pennsylvania cannot be ruled out. Thunderstorms are
slightly less likely at this time given increased cloud cover
and lower instability of the early morning hours, but cannot
rule out a thunderstorm across far southeastern Pennsylvania
with some elevated instability. After the cold frontal passage,
cooler and less humid air will rush in by Sunday morning.

All medium range guidance supports fair weather and above
average temperatures Sunday into early next week with upper
level ridging indicated over the northeast. At the surface, high
pressure is progged to build over Pennsylvania Sunday, then
pass off of the east coast early next week. The return southerly
flow should result in an uptick in heat/humidity early next
week.

Diurnally driven showers and thunderstorms appear possible on
Monday based on recent GEFS plumes but guidance continues to
suggest a better chance of precipitation on Tuesday with the
arrival of a cold front during the afternoon hours. There is
still a fair amount of uncertainty with regards to the cold
front clearing eastern Pennsylvania later on Wednesday, so have
retained PoPs through this timeframe. Towards the end of the
period, deterministic guidance begins to outline high pressure
influence into the region; however, given so much uncertainty
earlier in the forecast period have opted not to deviate from
NBM PoPs at this time.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
VFR cu field continues to expand gradually with varying amounts
of alto cu and cirrus above.

Deep convection has been largely capped across much of Central
and Southern PA so far where 700 MB temps (10 KFT MSL) were
quite warm and ranged from 8-9.5C. Mid level WSW flow is
advecting even slightly warmer air aloft across the Central
Ridge and Valley Region of the state.

Slightly cooler mid level temps across the northern Mtns and
the presence of a slow moving cfront was resulting in a
gradually growing area of showers, but so far no lightning in
Central/Northern PA.

A few well-defined outflow boundaries are surging south from the
Nrn PA convection and that should help to initiate additional
showers and isolated TSRA further south later this afternoon
into early this evening with some very brief MVFR CIG and VSBY
restrictions possible. Don`t think that the coverage of any
reductions or the threat for gusty winds will be great enough
to include as a tempo group at this point, but will monitor the
evolution of the convection as we head into the period of peak
heating and areal coverage of convection as the cfront and
outflow boundaries sage south toward the I-80 corridor.

A majority of the convective activity will be out of the region
by 03Z Friday. Expect patchy fog to develop across the region
overnight tonight with reductions to visby and cigs likely
Friday morning.

Outlook...

Fri...Sct TSRA w/ possible impacts; Mainly PM w/ highest
coverage across the west.

Sat...Sct to Nmrs TSRA areawide, highest chances SE in PM.

Sun...AM fog possible; otherwise, no sig wx.

Mon...SHRA possible SE late PM; otherwise, no sig wx.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...Lambert/Banghoff
NEAR TERM...Lambert/Banghoff
SHORT TERM...Lambert/Fitzgerald/Banghoff
LONG TERM...Fitzgerald/NPB
AVIATION...Lambert/Bowen