fxus61 kphi 271034 

Area forecast discussion 
National Weather Service Mount Holly New Jersey 
634 am EDT Mon Mar 27 2017 

northeastward moving low pressure from the upper Great Lakes to 
Missouri will pull a warm front northward through the Delmarva and 
southern New Jersey today. The warm front should stall across 
southeastern Pennsylvania and central New Jersey, awaiting the 
passage of the Missouri low, through New Jersey, Tuesday evening. 
Thereafter, Canadian high pressure will build into our area 
Wednesday and Thursday. Low pressure in the Ohio Valley on Friday 
will move east and off the mid Atlantic coast on Saturday. Weak 
high pressure should follow on Sunday. 


Near term /until 6 PM this evening/... 
few minor tweaks at 630 a.M. Added coastal Atlantic County to 
dense fog advy. It was mistakenly left out earlier. Also dropped 
Sussex Delaware from advy since vsbys there have improved. Minor 
downward adjustment to temps in Metro Philadelphia and N/W for 

A slow moving warm front will be tracking across the area today. 
Our forecast has trended slower with the frontal passage and 
includes a slower trend in improvement (sunshine) behind the 
front and slightly lower temperatures for today. We have issued 
a dense fog advisory for Delmarva and parts of southeast/PA and srn New Jersey 
for the early part of the morning, since the visibilities continue to 
trend downward and are 1/2 sm - 3/4 sm attm. The onshore flow 
will continue in these areas until the front passes later today. 
The low clouds and areas of dense fog will be over the area 
thru the first part of the morning before a S to north slow 
improvement trend is expected today. Areas across the Lehigh 
Valley and southern Poconos may see improvement late this 
afternoon. Confid in the north/west areas is limited attm. The low 
which the warm front is attached to is weak, so it's possible 
that the topography of these areas may play a factor slowing the 
motion. Highs today are expected to reach the upper 60s/low 70s 
over south New Jersey and Delmarva, low/mid 60s in central New Jersey , Metro 
Philadelphia and Lehigh Valley and 50s north of that. 


Short term /6 PM this evening through 6 am Tuesday/... 
hopefully the warm front will have moved far enough away to have 
a lull in the precip for the evening. More showers will arrive 
overnight as the next short wave approaches from the W/SW. Pops 
increase back to the chc range overnight, favoring slightly the 
srn New Jersey and Delmarva areas. Mild with lows in the low 50s S/east and 
mid/upper 40s N/W. Light winds expected. 


Long term /Tuesday through Sunday/... 
500 mb: a split flow regime will continue across the United 
States during this period. A short wave crosses Pennsylvania 
Tuesday and then phases with a northern stream system into a 
closed low over the Maritimes Wednesday night. The next southern 
stream short wave in the pipeline over the Mississippi Valley 
Thursday night, weakens eastward off the mid Atlantic coast 
Saturday. Yet another southern stream short wave will be moving 
eastward from the lower Mississippi Valley early next week. 

Temperatures: calendar day averages should be about 5 degrees above 
normal Wednesday, near normal Thursday and normal to 5 degrees 
below on Friday, and then normal or several degrees above on 
Saturday and Sunday. 

Forecast basis: 50 50 blended 00z/27 GFS/NAM MOS Tuesday- 
Wednesday, then 00z/27 mexmos Wednesday night-Thursday and 
thereafter wpc guidance Thursday night-Sunday. 

The dailies... 

Tuesday...the position of the warm front Tuesday will probably 
result in a wide range of temperatures across our area with 
large error potential I-78 region northward where global models 
are warm but the ever improving 12km NAM and its downscaled 3km 
temperatures, are much cooler. Dreary north of that warm front 
with patchy dense fog possible and a likelihood of heavy showers 
during the afternoon and evening as the Missouri low cuts east 
along the Mason Dixon line, heading for a New Jersey seaward exit 
Tuesday evening. There could be afternoon and evening 
thunderstorms with this short wave. Within the warm sector in 
advance of the cold front, we expect sunshine to break out south 
of the warm front with temperatures reaching the upper 70s. A 
low- level moist tongue will also be in place across this area, 
ml cape (400-800j), along with negative lifted and showalter 
indices, within an environment characterized by poor lapse rates 
and weak shear. Thunder is expected but despite the warm 
front,right now severe is not. Do follow later phi and Storm Prediction Center 
discussions. Precipitable water values around 1.25 inches, 
should yield some heavy showers near the path of the low. 
Movement should be east northeast and there could be some 
training and backbuilding along the warm front so that this may 
need to be monitored for narrow bands of poor drainage street 
flooding rain Tuesday afternoon/evening. Winds light northeast, 
north of the warm front (near I-78) and southerly in the warm 
sector Delmarva and S New Jersey. Confidence: above average except the 
location of the warm front and the associated temps near the 
warm front. 

Tuesday night...showers still may be heavy in the evening (still 
thunder possible?), Then end from west to east late and the 
wind turns north everywhere. Confidence: above average. 

Wednesday...a gusty northerly flow to 25 mph of cold air 
advection. Dry. Confidence: above average. 

Thursday...a sunny start, then increasing clouds and cool ahead 
of the next storm system. Light wind. Confidence: above average. 

Friday...increasing model consensus that this actually looks 
like a decent little mid Atlantic coastal storm with one half to 
1.5" of qpf, and currently, only a slight chance of a little 
wet snow near and north of I-80 since models have trended 
slightly northward. 

Friday evening tides may exceed the minor advisory threshold 
along the New Jersey Atlantic coast provided the sfc low passes south 
of Acy. 

Overall Friday confidence: above average., 

Saturday...precipitation(mainly rain) ends, but clouds may 
linger since there doesn't seem to be much in the way of high 
pressure following the low. Confidence: average 

Sunday...considerable cloudiness. Low confidence on what might 
transpire this day. 


Aviation /12z Monday through Friday/... 
the following discussion is for kphl, kpne, kttn, kabe, krdg, kilg, 
kmiv, kacy and surrounding areas. 

The latest issuance of the tafs are somewhat more pessimistic with 
regards to how fast the transition back to MVFR then VFR occurs 
today. Warm fronts are usually known for their stubbornness for 
improving conditions and this seems to be the case for today. A 
decent SW flow aloft argues for the fact that when things begin to 
improve, it should happen quickly, but what time that happens is the 
problem for the day. The showers that the models show arriving this 
morning may mix up the low clouds and fog enough to get the process 
going. Winds will be Erly prior to the warm front the SW behind it. 
VFR will probably be across the del valley and Delmarva/south New Jersey 
this afternoon, conditions may remain MVFR across krdg/kabe longer. 
Probably a period of VFR this evening, the more low clouds/showers 
late tonight. 


Confidence: above average all days. Exception being the details of 
the warm frontal position Tuesday and timing of poorest conditions 
where there is less than average confidence. 

Tuesday...IFR or MVFR conditions Tuesday morning at all taf sites, 
with improvement to VFR by afternoon at all but Abe, rdg, and ttn 
where IFR may may linger all day. Showers become widespread in the 
afternoon/evening with light northeast flow at Abe, rdg, ttn and 
light south or southeast flow elsewhere. Isolated tstms expected. 

Tuesday night...MVFR possible early in showers, otherwise becoming 
VFR late. Winds becoming north everywhere late. 

Wednesday...mostly VFR. North wind gusts 20-25 kt. 

Thursday...VFR. Light north wind. 

Friday...IFR or MVFR conds in periods of rain. Mostly east wind 
ahead of the low. 


we will continue with the Small Craft Advisory flag for seas around 5 ft seas today. 
The Erly flow will be weakening with time as the warm front slides 
northward today. I could see the flag being dropped earlier, but 
we will keep it for now. Dense fog on the waters this morning with 
the warmer and more humid air arriving across the cold water. 
Scattered showers later this morning may stir vsbys up. More sct 
showers expected tonight. Patchy fog tonight. 


Tuesday...fog may be marine hazard? Otherwise no Small Craft Advisory expected at 
this time. Thunderstorms could occur, especially late in the 
day. Confidence: average. 

Tuesday night...Small Craft Advisory northerly flow may develop late. Confidence: 

Wednesday...northerly flow Small Craft Advisory. Confidence: above average. 

Thursday...no headlines. Confidence: above average. 

Friday...Small Craft Advisory potential as low pressure heads for the mid 
Atlantic coast. Confidence: above average. 


Georgetown 3/28 rer Max is 80 set in 1960. 

Phl temperatures continue to project near 1 degree f below 
normal for the month as a whole. 


knel appears to be reading 10f too warm the past couple of 


Phi watches/warnings/advisories... 
PA...dense fog advisory until 10 am EDT this morning for paz070-071- 
New Jersey...dense fog advisory until 10 am EDT this morning for njz013-014- 
Delaware...dense fog advisory until 10 am EDT this morning for dez001-002. 
Maryland...dense fog advisory until 10 am EDT this morning for mdz008-012- 
Marine...dense fog advisory until noon EDT today for anz430-431-450>455. 
Small Craft Advisory until 7 PM EDT this evening for 


near term...O'Hara 
short term...O'Hara 
long term...drag 

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