Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS61 KPHI 240924

Area Forecast Discussion
National Weather Service Mount Holly NJ
424 AM EST Sat Feb 24 2018

A warm front will move slowly northward tonight 
into Sunday. A low pressure system will cross to the north of our 
area with its attendant cold front moving through Sunday night. High 
pressure will build into the region through midweek. A backdoor cold 
front will drop down through the region on Wednesday, followed by an 
area of low pressure crossing the area Thursday and perhaps 
intensifying along the coast early Friday.


Several changes were made to the forecast today. Fog is getting
dense in Chester County and the Maryland Eastern Shore. Have 
issued a dense fog advisory through 8 am for these areas, and 
there is at least some chance the advisory may need to be 
carried eastward into Delaware given short-term model forecasts 
and recent trends in DOV/GED surface observations. Visibility 
should improve by mid to late morning, but patchy fog and even 
some drizzle look to remain through much of the day across the 

This leads to concern number two, which is the forecast highs today. 
Inherited forecast simply looks too warm for the given pattern. 
Despite potential for rising cloud bases (albeit gradually) during 
the day, the overcast will very likely stick around. Though it has 
been a warm February, it is still February -- and with my 
pessimistic sky forecast today, I am awfully skeptical temperatures 
will reach the upper 50s in portions of SE PA/S NJ.  Generally, I 
lowered temperatures 3-5 degrees across the area, and I am wondering 
if this is enough, especially if this morning's observations agree 
with the colder NAM. Forecast temps may require more fine-tuning in 
future updates, but I felt that starting the trend downward was 

Several midlevel perturbations move northeastward into the northern 
Mid-Atlantic today, and with favorable positioning of the northern-
stream 250-mb jet max (right-entrance region dynamics in play) 
combined with low-level isentropic ascent along/north of a 
baroclinic zone stretching zonally across the region, periodic bouts 
of rain look to occur in the CWA beginning this afternoon, 
especially north of the Mason-Dixon Line. There is still some 
variability in timing of the precipitation, so I kept hourly PoPs in 
the 50-80 percent range for the afternoon until there is more solid 
agreement amongst the hi-res models. Nevertheless, rain is a pretty 
good bet in PA/NJ this afternoon with diminishing chances in 
southern MD/DE. QPF is generally a quarter inch or less through 00Z 
Sunday, though amounts may be a little higher than that in the 
Poconos and Lehigh Valley if the more aggressive models are


One round of rain should be diminishing and moving away from the 
region during the evening hours, but conditions will likely once 
again be favorable for low clouds, fog, and drizzle during this 
time. Trended PoPs downward during the evening hours, but maintained 
categorical PoPs after midnight as the next round of rain is 
expected to move in. Kept mention of fog through the night, but 
generally kept drizzle confined to the evening hours, as steadier 
rain is more likely after midnight, especially north of the Mason-
Dixon Line.

This next round of rain will occur as yet another vort max 
approaches the region in midlevel southwesterly flow between 06Z and 
12Z Sunday. Isentropic ascent poleward of the quasi-stationary 
baroclinic zone will allow for precipitation to break out again 
(with help from the steady 250-mb jet max to our north). The 
enhanced ascent provided by the stronger vort max, the slowly 
poleward-moving warm front, and the considerable upper-level 
divergence aided by the jet max will likely lead to more formidable 
rainfall totals during this period. Generally, amounts should be 
approaching a half inch to an inch north of the Mason-Dixon Line by 
night's end with lighter amounts to the south. Fairly decent 
agreement among hi-res models on QPF, so confidence is slightly 
above average.

Temperatures will not fall too much as the warm front makes its move 
(slowly) northward, overcast conditions continue, and warm advection 
curtails nocturnal cooling. Forecast lows are in the mid to upper 
30s in the Poconos and northwest New Jersey to the mid to upper 40s 
southeast of the Fall Line. Will need to keep an eye on temperatures 
in the Poconos, as models tend to have a warm bias in this area 
north of warm fronts, and even a couple degrees cooler could mean at 
least a chance for some isolated slick spots. For now, chances are 
too low for mention.


A progressive mid-level flow pattern will provide changeable weather 
for the Sunday through Friday time period, with normal to slightly 
above normal temperatures through most of the period. 

A warm front associated with a low pressure system moving NE through 
the Great Lakes region will lift north Sunday morning before its 
associated cold front pushes east during the afternoon and evening 
hours. The passage of this cold front will bring an end to any 
lingering rain and rain showers. As precipitation from this system 
ends Sunday night,rainfall totals are still expected to range 
between 1 and 2 inches. This will cause ponding of water on roadways 
and in poor drainage areas, and rises on small stream, but no 
significant stream and river flooding is anticipated at this time, 
especially given the longer duration of the event. Please see hydro 
section below. 

High pressure from the Tennessee River Valley is then expected to 
build east, bringing dry weather to the region from late Sunday 
night through most of Wednesday.  Cooler temperatures are expected 
Monday into Tuesday with a northwesterly flow (but still above 
normal), followed by warmer temperatures on Wednesday as a 
southwesterly flow once again establishes itself over the area. 
Another cold front is expected to cross the area Wednesday night but 
with limited moisture, precipitation amounts will be light.  A 
potentially more significant rain storm could move in later Thursday 
and Friday as low pressure develops near the mid-Atlantic coast. 
Some light snow is even possible on Friday in the far north with a 
northwest flow as the system moves NE away from the area.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

This morning...IFR to LIFR conditions in very low clouds and fog, 
with the densest fog southwest of Philly. CIGs should slowly rise 
after daybreak but may remain IFR for much of the morning. Timing 
the improvement in CIGs/VSBYs is very uncertain, but current 
thinking is that this may be slower to occur than previous forecasts 
indicated. Winds generally southwest under 10 kts, becoming more 
variable after daybreak. High confidence through daybreak, but low 
confidence thereafter.

This afternoon...Some improvement to MVFR or even VFR possible, 
especially at ACY/MIV, but periods of rain will likely move in from 
the west this afternoon. Sub-VFR VSBYs/CIGs in the steadier rain are 
likely. Winds becoming northeast under 10 kts. Moderate confidence.

Tonight...Prolonged sub-VFR conditions likely as fog/drizzle are 
expected this evening followed by steadier rain after midnight. 
Winds generally east or northeast under 10 kts. Moderate confidence.


Sunday...IFR conditions expected with limited improvement 
through the day. Confidence is low that we will return to VFR during 
the day. East to southeast winds will become southwest to west by 
the afternoon. Moderate confidence.

Sunday night...Improving to VFR through the night. Rain showers will 
taper off through Sunday evening and conditions are expected to 
improve. High Confidence.

Monday through Tuesday...VFR conditions expected. West to 
northwest winds becoming gusty to around 20 knots on Monday. 
High confidence.

Wednesday...VFR conditions expected. West to southwest winds, 
generally around 10 knots or less, becoming more south to 
southeast late. High confidence.


Generally sub-advisory winds/seas expected through this evening. 
However, winds will begin to increase on the northern/central NJ 
coastal waters late tonight as east to northeast winds begin to 
crank up in response to an approaching storm system. Have issued a 
small craft advisory for these areas beginning at 3 am Sunday.

Another concern is visibility restrictions this morning. Dense fog 
is occurring off the Maryland coast, and surface visibilities
are now deteriorating in Delaware and in coastal southern New 
Jersey. As a result, a dense fog advisory was issued for 
Delaware Bay and the Atlantic coastal waters of southern New 
Jersey and Delaware through 10 am.

Visibility restrictions may continue most of the day, though
visibilities may improve somewhat, and rain chances will 
increase this afternoon and tonight, especially off the NJ 

Saturday night...Sub-advisory conditions are expected on the 
area waters. 

Sunday and Sunday night...Seas are expected to build and exceed
5 feet on the ocean on Sunday. Winds also pick up through the 
day Sunday with gusts of 25 to 30 knots possible. Conditions will 
improve late Sunday night. A Small Craft Advisory has been 

Monday through Wednesday...Sub-advisory conditions are expected
on the area waters.


Periods of rain through the weekend could result in minor and 
poor drainage flooding. The heaviest rain is expected Saturday 
night and Sunday, when up to additional inch of rain will fall. 
By the time the rain ends Monday morning, the region will have 
received between 1 and 2 inches of rain since Thursday, with the
highest amounts falling over the northern third of the forecast
area. By Sunday night and Monday, there will be rises on areas 
rivers and streams, with some possibly approaching bankful.

Of note will be how much rain falls over the Susquehanna, as 
there is the potential for inconvenience flooding below the 
Conowingo Dam on Monday.


**Top 3 warmest February on record and top 10 wettest February
on record.**

February projected climate ranking as of Noon today based on 
our mid shift fcst through the 28th and mins this morning; and 
for rainfall, amounts through Noon today. RDG and TTN not 
included due to too much missing data. 

ABE #3 warmest 38.2 or 7.5F warmer than the 30.7 norm.

ACY #1 warmest 43.0 or 7.7F warmer than the 35.3 norm. Tied 
with last year. #2 40.6 in 1954

ILG #3 warmest 41.6 or 6.6F warmer than the 35.0 norm. 

PHL #3 warmest 42.0 or 6.3F warmer than the 35.7 norm.

Water equivalent monthly pcpn

PHL ranked #10 with 5.28" (0.21 as of Noon daily)
ILG ranked #8 with 5.49" (0.49 as of Noon daily) 
ABE ranked #16 with 4.29" (0.07 as of Noon daily)

ACY ranked #2 with 6.12" (0.28" as of Noon daily) 
Wettest is 6.50-2010.  Our expectation is that ACY will exceed the 
previous all time record rainfall for February, by the end of the 


PA...Dense Fog Advisory until 8 AM EST this morning for PAZ101-102.
MD...Dense Fog Advisory until 8 AM EST this morning for MDZ008-012-
MARINE...Small Craft Advisory from 7 AM Sunday to 7 AM EST Monday for 
     Dense Fog Advisory until 10 AM EST this morning for ANZ430-431-
     Small Craft Advisory from 4 AM Sunday to 7 AM EST Monday for 


Near Term...CMS
Short Term...CMS
Long Term...Miketta

National Weather Service Glossary of Abbreviations