387 
FXUS61 KPHI 191650
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1250 PM EDT Tue Oct 19 2021

.SYNOPSIS...
High pressure will drift slowly eastward through the southern
and eastern U.S. through Thursday. A cold front will move
through the Mid-Atlantic Thursday night and Friday, with low
pressure developing offshore Friday night and Saturday. High
pressure will build back into the eastern U.S. by the beginning
of next week.
&&

.NEAR TERM /THROUGH TONIGHT/...
An upper low over the Canadian Maritimes will move only slowly 
eastward during the next 24 hours, with a broad surface high 
shifting slowly southeastward through much of the southern and 
eastern U.S. The northern Mid-Atlantic will remain at least on 
the fringes of steady west-to-northwest surface flow today, but 
with midlevel ridging impinging on the region as well as a 
subtle backing of the winds today versus yesterday, conditions 
will be noticeably different today. The biggest change will be 
to the low-level advection pattern, with only modest cold 
advection becoming mostly neutral by late in the day. The 
surface flow will also become more favorable for downsloping. 
Combined with increasing heights/thicknesses from the 
approaching ridge, temperatures should be noticeably warmer 
today, likely aided by less persistent cloud cover as well. 
Forecast highs are around ten degrees higher than those seen 
yesterday. Given the expected persistent pressure gradient 
today, west to northwest winds may gust to 20 to 25 mph or so 
late this morning and this afternoon.

A subtle perturbation will progress east-southeastward within
broad midlevel ridging approaching the eastern U.S. tonight.
Subtle large-scale lift in advance of the perturbation combined
with weak low-level warm advection should prevent substantial
cooling tonight. Lows are forecast to be about ten degrees or so
warmer than this morning's readings. One caveat, however, is if
winds decouple quickly during the evening hours (and before 
increased cloud cover occurs). Should this happen, temperatures 
may drop rapidly near/after sunset, especially in the Pine 
Barrens.
&&


.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The forecast for our region looks quite nice for this period,
with the Mid-Atlantic remaining on the northern periphery of a
broad surface high in the Southeast into the adjacent western
Atlantic. The upper-level pattern will feature two separate
perturbations that will modestly interact as they progress 
eastward from central to eastern portions of North America 
during the period. The northern perturbation, a strong upper low
digging south-southeastward through Ontario Wednesday and 
Wednesday night, will pivot eastward into Quebec on Thursday. 
The second perturbation will migrate eastward from the central 
Plains to the Saint Lawrence Valley through the period. This 
second perturbation will weaken as it impinges on downstream 
ridging in the western Atlantic (a common evolution seen the 
past couple of months). Thus, the attendant cold front extending
south-southwestward from the progressive surface low near/north
of the Great Lakes region will undergo frontolysis as it 
approaches the local area Thursday night. QPF model to model 
looks light/scattered and primarily stays to the north and west 
of the CWA. The only exception appears to be the Poconos, Lehigh
Valley, and northwest New Jersey, where some slight-chance to 
chance PoPs are placed Thursday afternoon/night as the southern 
perturbation glances the region.

With downstream ridging in place from the aforementioned 
systems, Wednesday and Thursday should be warm, with highs and 
lows several degrees above seasonal averages. It is possible the
current forecast is a little conservative, especially for 
Thursday, with highs approaching 80 not out of the question in 
portions of the urban corridor and in Delmarva.
&&


.LONG TERM /FRIDAY THROUGH TUESDAY/...
Models continue to struggle with the large-scale evolution
during this period, with large run-to-run changes still noted
with the deterministic simulations. Though the overall forecast
continues to look fairly tame for our region during this period,
confidence in the details is lower than average.

The main sticking point to forecast predictability appears to be
tied to the languishing upper low in southeast Canada and its
potential interactions with fast-moving southern-stream
perturbations across the northern/central U.S. These southern-
stream perturbations, in particular, feature little run-to-run
consistency, and given their origins in the notoriously
challenging northeast Pacific, suspect model variability will
continue to be quite high in the Days 4-7 period for the next
several days given that deep troughing will likely persist there.

The overall setup appears favorable for our region to be
relatively unscathed, however, since the upper low will be close
enough to prevent strong cyclogenesis from occurring in close
proximity to the area (the more favorable area being in the
western Atlantic). Thus, as one perturbation moves through the
Northeast Friday night and Saturday, most of the sensible-
weather impacts will likely be to our east. Nevertheless, there
will likely be a period of time in which at least light
precipitation occurs in our CWA, most probably some time in 
this Friday night to Saturday time window. This is when PoPs are
highest during the medium-range period, with conditions 
expected to dry out Saturday night and Sunday.

However, we will need to watch the evolution of the upper low in
southeast Canada as well, since it may pivot far enough
southward to bring another bout of light precipitation by early
next week. Confidence in this scenario is low, given the widely
disparate model solutions. Notably, however, there would be
limited impacts from this system to our area given the
moisture-starved appearance to its proximate environment.

The more noticeable impact from the evolving pattern is expected
to be a significant cooldown, with cold high pressure eventually
settling into the region. Though the cooldown will likely be
gradual (i.e., a few degrees each day), highs on Monday are
forecast to be about 20 degrees lower than highs on the previous
Thursday.
&&


.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY and surrounding areas.

This afternoon...VFR with west to northwest winds around 10 kt 
with gusts up to 20 kt possible at times. High confidence.

Tonight...VFR with light west winds. High confidence. 

Wednesday...VFR with west winds around 10 kt with occasional
gusts to 20 kt possible. High confidence.

Outlook...

Wednesday night through Thursday night...Mainly VFR with light 
and variable winds Wednesday night becoming southwest 5 to 10 kt
on Thursday. Some showers possible northwest of the Philly 
terminals Thursday night. Moderate confidence.

Friday...Mainly VFR with west or southwest winds 5 to 10 kt.
Moderate confidence.

Friday night and Saturday...Restrictions possible with a chance
of showers. Winds generally light northwesterly. Low confidence.

&&

.MARINE...
The Small Craft Advisory for the Atlantic coastal waters 
continues through tonight as winds should ramp up again later 
this afternoon into this evening. 

For Delaware Bay, conditions should generally be just below 
advisory criteria through tonight, though a few gusts to 25 kt 
may occur at times.

Outlook...

Wednesday and Wednesday night...Sub-advisory winds/seas
expected.

Thursday and Thursday night...Southwest winds approaching
advisory criteria Thursday afternoon and night, with seas
building to around 5 feet.

Friday through Saturday...Sub-advisory conditions expected. A
chance of showers.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ450>455.

&&

$$
Synopsis...CMS
Near Term...CMS/Fitzsimmons
Short Term...CMS
Long Term...CMS
Aviation...CMS/Fitzsimmons
Marine...CMS