Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Gray - Portland, ME (GYX)

                            
231 
FXUS61 KGYX 210432 AAD
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
1132 PM EST Tue Feb 20 2018

.SYNOPSIS...
Record breaking warm temperatures are expected across much of
the region on Wednesday as deep southwesterly flow continues
across the region ahead of an approaching cold front on
Wednesday. This front will move through the region Wednesday
night with cooler and drier air filtering into the area. Low
pressure will pass south of the region with a chance for some
light rain and snow over southern areas before another area of
low pressure arrives late Friday and Friday night bringing a
wintry mix to the region. The unsettled pattern looks to
continue through the upcoming weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Update...Minor changes to account for latest observational
trends.

Previous discussion...
High Impact Weather Potential: 

* Dense fog likely over areas with snowpack through tonight. 

* Elevated Ice Jam potential due to snowmelt and recent rainfall.  

Pattern:  Early afternoon water vapor imagery reveals amplified 
trough/ridge pattern across the United States with an impressive 
moisture plume /PWATS +5-6 sigma/ between these features directed 
from the Gulf Coast north and east up the Ohio Valley...through the 
Great Lakes and across far northern New England.  A look at the last 
4 RAOBs at PWM reveals impressive warming aloft...with a slower 
warming trend at the surface...with some resistance to the warming 
aloft given a cold surface high over the Canadian maritimes...that 
continues to feed cool low level air into parts of northern New 
England.  A look down at the surface pattern reveals low pressure 
moving north and east into the western Great Lakes. Through the near 
term forecast period...this low will continue to trek north and 
east...with its attendant warm front lifting north through the 
forecast area /at least aloft/. Forecast concerns center around 
precipitation potential surrounding this front as well as dense 
fog potential as increasingly warm/moist air flow over a 
retreating snowpack. 

Through this evening: A tale of two forecast areas late this
afternoon with the warm front across southern NH...with
mid/upper 60s to the south...and low clouds...some fog...and
upper 30s/lower 40s to the north. Front will stop making
northward progress through this evening....with fog north of the
boundary beginning to thicken after sunset.  

Tonight: Low level jet forcing continues to transport increasing
warmth and moisture above low level cold dome this evening and 
overnight as warm front lifts north. The northward lifting warm 
front should bring showers over my far northern zones to an end.
To the south...the warm/moist advection will lead to an ideal 
setup for fog with moisture increasing with height in the first 
1kft. Essentially...where the warm front hasn/t reached today 
appears likely to see renewed fog this evening which is expected
to become dense given more moisture aloft tonight. Thus...will 
go ahead and issue a dense fog advisory for these areas and let 
evening shift adjust as fog becomes more widespread this 
evening. Otherwise...a mild night is in store...with lows only 
falling back to the 40s to around 50 in NH and upper 30s/lower 
40s across western Maine.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
High Impact Weather Potential: 

* Record high temperatures likely on Wednesday

Pattern: Low pressure east of James Bay will continue to move east 
through the day Wednesday as high pressure builds east into the 
Great Lakes region.  A trailing cold front from this low will push 
through region Wednesday evening with high pressure associated 
drier airmass settling into the region Wednesday night. Primary 
forecast concerns will center around the potential for daily 
/monthly?/ record highs in the warm sector on Wednesday. 

Wednesday: While much of the area will again dawn to low clouds and 
fog...the warm front will have advanced to a location north of the 
forecast area.  With strong low level jet just off the deck...expect 
the clearing process to be much more rapid than today and with
T9s warming to +12-16C /and noting temperatures over MA today/
expect 70 to be in reach over southern NH and southwestern ME.
Bigger question will be along the ME coast from Portland north
and east where somewhat of an onshore component may limit highs
somewhat. Even here...however...expect a more westerly component
in the low level flow to develop by mid/late afternoon as the 
cold front approaches from the north and west...likely allowing 
coastal locations to warm /although probably not as warm as 
areas inland/. Given what occurred today...leaned more heavily 
towards the warmer MAV guidance. See climate section below for 
details on record highs. Shower chances will return to the 
mountains in the afternoon...although given weak convergence 
along this boundary and westerly flow...see little in the way of
shower activity making it south of the mountains. 

Wednesday Night: Cold front will be along the coastline at 00Z 
Thursday...and slowly sink south overnight while stalling south of 
the region as it becomes increasingly parallel to the mid and upper 
level flow.  The cold advection is not particularly impressive 
although surface dewpoints will dramatically fall from evening 
values in the 40s and 50s back into the teens and 20s by daybreak 
Thursday. Northwesterly winds should remain coupled through the 
night given the aforementioned cold advection and a collocated 
isallobaric component. So...while a significantly colder night is 
expected...lows will still remain above seasonal norms...teens north 
to lower/mid 30s along the coast.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Cold air continues to drain in from the north on Thursday with
the surface high building in from the west. Highs will make it
into the low 40s south of the mountains where a downslope
component keeps things warm, especially in the morning.
Temperatures may fall during the afternoon as the cold air
continues to pour in. The first in a series of weak waves moves
across southern New England on Thursday and may spread some
light rain into southern New Hampshire during the day, with snow
possible on the northern edge as temperatures continue to cool.
Precipitation will be quite light. High pressure moves over the
area Thursday night allowing some good radiational cooling
conditions. Dewpoints in the teens suggest we will cool into the
teens to perhaps single digits in the colder valleys.

Fast moving wave crosses the area Friday night with the next
round of precipitation expected. This will be primarily of the
warm advection style precipitation as a warm front moves
through. Southeast flow ahead of this feature, though, suggests
that cold air damming will be a factor especially in interior 
western Maine. Much of the precipitation will fall as
temperatures aloft are still cold enough for snow, but as the
warm front moves through we may see some areas of freezing rain
before it ends. Much warmer on Saturday behind the warm front
with temperatures in the mid to upper 40s and a few 50s. This 
does not last too long as a cold front drops in from the north
Saturday night bringing the colder air back in.

The most robust of the waves this weekend will be the one
arriving on Sunday. This will be tracking out of the Texas
Panhandle and into the Great Lakes, with another surge of warm,
moist air ahead of it. As the broader trough arrives in our
area, the surface low will occlude with secondary low developing
near the coastline, complicating the precipitation type
forecast. A sustained easterly low level flow is expected ahead
of the wave which is a strong signal of cold air damming being 
at play even into southern New Hampshire. This will likely keep
temperatures near freezing even as temperatures aloft try to
warm with the parent low moving to our northwest. But with the
secondary low forming near the coast, we may hang on to deep
enough cold air to keep much of the precipitation as snow, but
if the warmer air moves in aloft we will see a switch to
freezing rain.

High pressure builds in Monday and Tuesday with temperatures
near seasonal normals.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Short Term...

Summary: Deep southwesterly flow will continue to transport 
moist air across the region through tonight and Wednesday before
a cold front pushes through the region Wednesday evening. High 
pressure will build into the region Wednesday night. 

Restrictions:  Highly variable conditions early this afternoon with 
fog/low clouds scattering out over southern NH...but showers/fog
continuing at HIE-AUG. Expect a bit more improvement through 
mid afternoon...then deterioration this evening with widespread 
VLIFR/LIFR in fog/low stratus except perhaps at MHT. Expect more
rapid improvement to VFR on Wednesday morning with VFR 
continuing through Wednesday Night. 

Winds: Light/variable winds /5-10kt southerlies LEB-MHT-CON-PSM/ 
remaining light overnight before becoming southwesterly around 10kts 
for the day on Wednesday...shifting northwesterly 5-10kts Wednesday 
night. 

LLWS: Southwesterly winds at 1-1.5kft will increase to around 40kts 
this evening...promoting LLWS through the night.  Winds aloft will 
strengthen through the day Wednesday which may prolong the LLWS 
threat despite some morning mixing. Any LLWS will come to an end 
Wednesday night as the cold front pushes through the area.  

Long Term...Several waves move through the region late this
week and this weekend with the potential for lower clouds and
precipitation with each of them. The first will primarily affect
southern New Hampshire on Thursday. The next brings snow and
rain on Friday. A larger system arrives on Sunday with more
widespread snow possible.

&&

.MARINE...
Short Term...
Southwesterly winds will increase on Wednesday /with seas also
building as a result/...renewing small craft headlines for 
Wednesday/Wednesday night. 

Long Term...Winds becoming northerly behind the front Wednesday
night and will be strong early before diminishing on Thursday.
Several waves move across the area this weekend, the strongest
being on Sunday. This could provide another chance for advisory
level wind speeds.

&&

.HYDROLOGY...
Flood watch continues unchanged with the forecast this afternoon
for the threat of ice jams over western New Hampshire. Rainfall
amounts have been light...but dewpoints continue to climb and
do expect a significant melt in the headwaters tonight and
through Wednesday. Lesser threat exists to the east where less
rain has fallen...there are less existing jams...and
temperatures through tonight are cooler.

&&

.CLIMATE...
Record high temperatures are expected across the region on 
Wednesday.

*** February 21 Records Temperatures***
Portland            Concord             Augusta 
59F - 1953          64F - 1872           52F - 1981

***Warmest February Temperatures***
Portland            Concord             Augusta 
64F - Feb 26, 1957   69F - Feb 24, 2017   60F - Feb 18, 1981

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Dense Fog Advisory until 7 AM EST Wednesday for MEZ007>009-
     012>014-018>028.
NH...Flood Watch through Wednesday evening for NHZ001>003-005-007.
     Dense Fog Advisory until 7 AM EST Wednesday for NHZ001-002-004-
     006-010-014.
MARINE...Small Craft Advisory from 5 AM Wednesday to 5 AM EST Thursday 
     for ANZ150>152-154.

&&

$$

NEAR TERM...Legro