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Area Forecast Discussion (AFD)

Issued by Caribou, ME (CAR)

FXUS61 KCAR 150410

Area Forecast Discussion
National Weather Service Caribou ME
1210 AM EDT Wed Aug 15 2018

A weak area of low pressure over upstate New York will slowly 
move over the area tonight. A cold front will then move through
the region later Wednesday with scattered showers and
thunderstorms expected during the afternoon. High pressure then
builds in for Thursday before the next low pressure system moves
in from the west Friday afternoon.

12am update: Upper level low is spinning towards the state and
will increase elevated instability in the coming hours. Will go
with mention of isolated thunder this morning and scattered
thunderstorms this afternoon into the evening. Although sfc-
based htg will not be impressive, it won't take much to develop
sufficient SBCAPE given the expected sfc dew pts and cooling
aloft. Bulk shear remains unimpressive during peak afternoon
heating hours, but the potential of some stronger storms
prompted mention of gusty winds and locally heavy rainfall for
this afternoon.

Previous discussion:
Upr low spinning near the Catskills in upstate NY with flooding 
contg acrs NY and PA and slowly shifting east into New England this 
aftn. Onshore moist flow conts acrs the CWA with some breaks in cld 
cvr opening up and allowing a few showers to dvlp acrs cntr sxns. 
Instability is meager but still exists acrs the region per latest 
SPC meso-page. Hv contd isold thunder mention acrs the area this 
aftn until loss of diurnal htg this evng. 

Expect best chc for showers drg the ovrngt wl be across the far 
north in area of upr lvl diffluence. Yet another upr trof diving 
into the nrn Plains this afternoon and this wl serve to kick the 
closed H5 low out ovrngt. 

Sfc cdfnt is located just south of James Bay and wl head south 
tonight. Moist and humid air wl be present for one more night with 
frontal bndry mvg twd a St. Lawrence river location by 12z tomorrow. 
Patchy fog along the immediate coast this evng wl give way to areas 
of fog aft midnight. Patchy fog wl impact rmndr of CWA late tonight 
into the mrng hrs. 

Expect that showers assoc with the front wl begin to makes its 
impact acrs the St. John Vly and the North Woods aft 12z. With 
elevated instability present isold thunder wl be possible drg the 
mrng, bcmg more sctd in the aftn. As far as svr chcs go, with marine 
layer present ovr the region this wl lkly limit svr potential. CAPES 
may top 1000 J/kg where substantial htg can take place but bulk 
shear values look to be on the weak side ovr most of our area, 
outside of far swrn zones. 

PW values of btwn 1.5-1.75 inches still makes locally hvy rainfall 
the biggest threat with the storms tomorrow aftn. Cannot rule out a 
strong wind gust tomorrow but no confidence to include enhanced 
wording in fcst attm. Cool and dry air begins to filter in to nrn 
areas by vry late in the aftn twd evng.

Showers and a few thunderstorms may be lingering over the area
at the start of the period, with the best chances over the 
eastern CWA. However, things should die down by midnight, and
any chance POPs or mention of thunder are out of the CWA by 
then. Expect some general clearing of skies Thu into Thu night
behind the front, though some cloud cover may linger early 
before mixing out in the afternoon. Some cloud cover could also
persist over the northeastern CWA, closest to the slowly 
departing cold pool and upper trough over the Maritimes. With a 
forecast for some more sunshine compared to the previous 
forecast, Thu highs were bumped up a degree or two. Despite the 
passage of the cold front, forecast 925mb temps in the mid to 
upper teens should allow for highs in the mid-70s to low 80s
with sunny skies and good mixing.

Thu night looks to be comfortable, with lows mainly in the 50s
across the CWA. Winds will shift onshore and moisture will be on
the increase on Fri as an area of low pressure approaches from
the Great Lakes. Some showers and a few thunderstorms may move
into the western CWA by the afternoon hours, but the better
chances will be Fri evening. Did put in a slight chance of
tstorms in the afternoon, becoming a chance of thunderstorms in
the evening hours. Also did put in some moderate and heavy rain
showers in the Wx grids starting 6pm Fri for areas with likely
PoPs and/or thunderstorm chances. Ultimately, any thunderstorm
chances and heavy rain wording will be dependent on the track of
the low. Currently, the forecast is for the low to go across
southern Maine and toward Nova Scotia, with t-storms/heavy rain 
along and somewhat north of the track.

The ECMWF came closer to the GFS and CMC timing for this low
pressure system - previously it had been slower by about 6
hours. Thus confidence on precip timing is increased, and low-
end categorical PoPs were introduced for the central Highlands
and Penobscot Valley in the 00z to 06z Sat time frame, which 
ended up matching well with GYX's PoPs. Based on the forecast
track of the low, t-storm chances were kept south of a
Moosehead-Topsfield line. Heavy rain showers were put in areas
with both t-storm chances and categorical POPs, with moderate
rain for other areas. There remains some disagreement with how
quickly precip clears out behind the low. The ECMWF has flip-
flopped on this point - it previously was one of the slowest to
clear precip, but now it pretty much has the CWA clear after 12z
Sat. Chance POPs were maintained over much of the CWA during the
day Sat, with low-end likely POPs along the coast, but a
downward tweak in POPs for Sat and early Sat night may be needed
if the trend toward faster clearing continues on future runs.
Behind this feature, Sunday into Mon will likely be quite
comfortable and feature decent sunshine. Cloud cover and
moisture may begin to increase again Tue, with rain chances on
the increase later Tue.

NEAR TERM: LIFR at BHB through 16z Wed with conditions to
improve to MVFR Wed afternoon. LIFR at BGR will improve to VFR 
by Wed afternoon. VFR at HUL north to FVE through 03z then will 
flirt with LIFR after 05z before rising to MVFR by 14z and VFR 
by afternoon. Conditions will lower in any afternoon showers and
thunderstorms. Locally strong and erratic wind gusts are 
possible in/near t-storms. 

SHORT TERM: Aside from some coastal fog and perhaps some patchy
onshore fog over land during the nights, mainly VFR conditions
are expected through Fri. A decrease to IFR conditions expected
Fri eve through Fri night with heavy rain and some t-storms
expected over the CWA. Improving conditions expected during the
day Sat, with VFR conditions Sat night into Sun.

NEAR TERM: Areas of fog will be the main impact through the day
Wednesday. Winds will be 15 kts or less with seas between 2-3

SHORT TERM: Dense fog will remain the main risk to safe
navigation later Wed thru Fri. Seas of 2 to 4 ft on Thu will
drop to 1 to 3 ft on Fri. Winds will remain gentle to moderate
through the period.


Near Term...CB/Farrar
Short Term...Kredensor
Long Term...Kredensor