676 
fxus61 kgyx 062011 
afdgyx 


Area forecast discussion 
National Weather Service Gray ME 
411 PM EDT Sat Sep 6 2008 


Synopsis... 
the remnants of Hanna will cross the Gulf of Maine late tonight 
and early Sunday morning. High pressure will build in behind the 
system Sunday afternoon and evening. The ridge of high pressure 
will build across Pennsylvania on Monday...influencing our weather 
as it drifts east Monday night. Low pressure tracking across the 
Great Lakes will pass to the north which will drag a cold front 
across the region late Tuesday. High pressure will build across 
New England Wednesday and Thursday. 


&& 


Near term /through tonight/... 
the main threat of course are the remnants of Hanna. 


Currently...the forecast area remains in the deep southwest flow 
ahead of the short wave to the west and Hanna to the southwest. 
Starting to see rain moving northeast ahead of Hanna in the Theta 
east rich low to middle levels. Also seeing some scattered convection 
developing in the quasi warm sector in the southern zones. 


Since the GFS and TPC tracks have been consistent...the 1200 UTC 
GFS was used for the forecast...as the mass fields look good. Base 
Don radar trends...brought the rain in this evening...and continued 
to mention heavy rain later this evening and overnight. Based on 
rainfall amounts in the middle Atlantic...still looking at 2 to 5 
inches of rain in the watch area. More on the rain and hydrological 
implications included in the hydrology section below. 


Again...since Hanna will be undergoing extratropical during the 
overnight into Sunday morning...would expect the strongest winds 
to remain on the east side of the storm. Based on this...would 
expect the strongest winds over land to remain below 45 miles per hour in 
gusts...and this could be generous...given the expected track. 
This does not seem to warrant a Wind Advisory at this time for 
Rockingham County and the southern Maine coast. Later shifts can 
monitor the wind field and adjust this as needed. 


Based on the high dew points and little airmass change tonight...lows 
should be close to the MOS consensus. 


&& 


Short term /Sunday/... 
given the track and speed of Hanna...the rain could cut off fairly 
quickly from west to east in the morning hours. The downslope 
behind the system should scour the moisture in the favored locations... 
but northern and eastern zones could hold onto showers a bit 
longer. After this...would expect partly to mostly sunny skies in 
the downslope. 


Winds will be at their highest in the morning as the remnants of 
Hanna pass. After this...the gradient drops off fairly quickly in 
the afternoon. Highs should be closer to a MOS consensus...as the 
GFS MOS may be a tad too high on the back side of Hanna. 


&& 


Long term /Sunday night through Saturday/... 
a weakening short wave will cross Sunday night. However given it 
is largely sheared out by the time it gets to US...and that it is 
a night time crossing...stick closer to the mav guidance Sunday 
night...keeping US dry outside of widely scattered showers. 


500 mb flow lifts in weak ridging aloft behind the 
shortwave...allowing high pressure to ridge north for Monday and 
Monday night. Use NAM/GFS MOS blend for temperatures through the 
period...with little adjustments needed. 


A strong cold front approaches the region Tuesday. 12z model 
guidance is significantly slower than some previous guidance. NAM 
is an outlier on the slow side...with the Gem by far the fastest. 
Prefer GFS/UKMET blend which falls between the two 
extremes...bringing the front through the region around 03z Wednesday. 
This later timing should diminish the severe weather threat for 
Tuesday...with the front passing during the evening hours. 
Still...wind fields are very strong for this time of year...with 
the GFS forecasting 65 kts at 850 mb...any sort of line segments 
would pose a wind damage threat. Thus far...instability is 
expected to be limited given the later timing...though it is close 
enough to bear watching as models focus in on the event. 


Beyond...12z GFS and Gem and 00z European in general 
agreement...though some minor timing differences do exist. Should 
see a strong Canadian high pressure system build in Wednesday and 
Thursday...with the next system approaching Friday. Only issue 
this may cause is the potential for freezing temperatures in the 
mountain valleys Wednesday night. Still way too off to pin down 
temperatures. For now hedge near MOS...though tweak up Thursday 
night and beyond due to MOS bias on lows. 


&& 


Aviation /19z Saturday through Thursday/... 
variable conditions this afternoon as holes developed in the 
stratus. This should end fairly quickly as the stratus ahead of 
Hanna fills back in. IFR/LIFR conditions are expected through at 
least 0900 UTC Sunday. Conditions will improve rapidly after 
this...and all terminals should be VFR shortly after 1500 UTC. 


Winds will be gusty with the passage of the remnants...and 30 knots 
winds are possible for time in gusts at kpwm and kpsm between 0800 
UTC and 1400 UTC. 


In the long term...scattered thunderstorms may impact aviation 
Tuesday afternoon and evening...otherwise expect predominantly VFR 
conditions Sunday night through Thursday. 


&& 


Marine... 
winds are near small craft levels in the southerly gradient on the 
back side of the strong high east of the Middle Atlantic States and 
Hanna to the southwest. The flow will begin to back as Hanna 
starts to approach. 


The GFS and TPC tracks are still very close and this was used for 
the timing in the forecast for tonight and Sunday. Hanna is 
already showing sighs of asymmetry in the wind field (with the 
strongest winds east of the center). Would expect this to continue 
and capped wind speeds with system at 45 knots...and this would be 
due to baroclinic processed as Hanna makes the transition from 
warm core to cold core as it passes. 


Would expect winds to drop back fairly quickly late Sunday morning 
and Sunday afternoon...and winds could be below small craft levels 
by late afternoon. However...seas will take time to sort itself 
out in the wake of Hanna...and seas should remain above 5 feet 
through Sunday evening. 


In the long term...Small Craft Advisory conditions are likely ahead of the cold 
front Tuesday...otherwise winds/seas should drop behind the front 
Wednesday and Thursday. 


&& 


Hydrology... 
only a few changes in the rationale for the flood potential for 
tonight and Sunday morning. Based on observations in the Middle 
Atlantic States...the expected amount of the rainfall was 
increased a bit (quite a few 5 to 6 inch rainfall amounts EST of 
District of Columbia this afternoon) to 3 to 5 inches. Also included a mention of 6 
inches near the coast in the low level convergence as the remnants 
of Hanna move across the Gulf of Maine late tonight and early 
Sunday morning. 


This much rain would result in street flooding and possible Road 
washouts...especially across Rockingham County and the southern 
half of western Maine. The Presumpscot River would also react 
quickly and could go into flood. Based on this...the Flood Watch 
continues where it was before. Sullivan County (nhz007) was added 
to the watch based on rainfall this morning and again this 
afternoon. 


Would expect the rain to end fairly quickly as the remnants of 
Hanna pull astride of Portland. However...it may take some time 
for the rain to run off...and the watch continues through late 
Sunday morning. 


&& 


Gyx watches/warnings/advisories... 
ME...Flash Flood Watch from 8 PM EDT this evening through Sunday 
morning for mez012>014-018>028. 
New Hampshire...Flash Flood Watch from 8 PM EDT this evening through Sunday 
morning for nhz004-006>010-013-014. 
Marine...Gale Warning until 12 PM EDT Sunday for anz150>154. 


&& 


$$ 


Long term...apffel 

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