NWS Technical Forecast Discussion

000
FXUS61 KBOX 050743
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
243 AM EST Fri Mar 5 2021

.SYNOPSIS...
Dry weather persists through the middle of next week. Colder
than normal air remains in control through the weekend. The
pattern then moderates to warmer than normal conditions for the
middle of next week.

&&

.NEAR TERM /TODAY THROUGH 6 PM/...

Low pressure sitting over Labrador and Eastern Quebec feeds cold air
advection into Southern New England today on gusty northwest winds.
Surface pressure gradient will be sufficient to maintain sustained
winds of 10-15 kt, while winds aloft in the mixed layer will be
20 to 25 kt...these would be available for gusts.

Speaking of the mixed layer, the ECMWF shows the layer reaching 925-
mb, while the GFS suggests 900-mb. ECMWF temps at 925-mb are
forecast at minus 9C to minus 11C, while the GFS at 900-mb has minus
11C to minus 13C. Converting these temperatures to a standard level
suggests max sfc temps of 26F to 35F. If mixing gets closer to 850-
mb in a few spots, the max temps in those places could reach 36F to
38F.

Cold pool moisture from the upper low will linger during the day,
and generate areas of clouds, especially north of the Mass Pike. The
surface air will remain rather dry with dew points in the single
numbers.

&&

.SHORT TERM /6 PM TONIGHT THROUGH 6 PM SATURDAY/...

Low pressure lingers over Eastern Canada into Saturday, maintaining
a surface pressure gradient over Srn New England although diminished
from the levels of today. There will also be lingering clouds from
the cold pool aloft. But with continued dry air at the surface,
expect dry weather through Saturday.

The wind may keep the air stirred tonight and prevent radiational
cooling. With dew points in the single numbers, there should be room
for temperatures to drop into the teens. Temps aloft will show
similar values to today. Therefore will forecast similar max temps
for Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Highlights

* The weekend will be dry and cold. Below average temperatures
  expected through Sunday. More seasonable temperatures on Monday.

* Little to no chance of precipitation over the forecast period.

* Warm-up on the way starting next Tuesday and extending late into
  the work week. 50s and even 60s are not out of the question for
  Wednesday.

Saturday night and Sunday...

With an upper level low still in place over Nova Scotia and the
Canadian Maritimes, several shortwaves will bring a series of cold
shots to New England this weekend. These cold spells are not expected
to be accompanied by any significant precipitation, but will keep
temperatures 5 to 10 degrees colder than average.

The upper level low is in sync with a deep trough that extends the
540mb isobar well into the Carolinas Saturday and Sunday. Gusty,
northwest flow associated with this trough will keep wind chills in
the 10s and 20s, especially Saturday, before the winds relent a bit
on Sunday. High temperatures on both Saturday and Sunday are
expected to range from the 20s north and west of our metropolitan
areas to the mid 30s south and east, extending across the Cape and
Islands. Guidance has come into agreement that this cold shot will
extend through the weekend before relinquishing its grip early next
week.

On Saturday night, conditions are ripe for radiational cooling with
weakening winds, clear skies, and a remaining snowpack north and
west of the I-95 corridor. To account for this low temperatures on
Saturday night were lowered using CONSMOS guidance.

The deep southern extension of the trough will keep us dry through
the weekend.

Monday thru Wednesday...

On Monday, the ridge that has been so dominant over the western US
begins to nudge into our area. Temperatures begin to level out
around average, in the upper 30s and low 40s, before the primary
wind direction switches from west/northwest to southwest, ushering
in more spring-like temperatures for mid-week. High pressure settles
over New England with this ridge.

Temperatures are likely to top out in the 50s on Tuesday with the
potential to reach 60 in several locations away from the immediate
coast of Massachusetts and Rhode Island on Wednesday. The Cape and
Islands will remain in the 40s to near 50 as SSTs are still hovering
around 40 degrees. Warm spring patterns that occur pre-"green up"
have a tendency to overperform compared to the highs forecasted by
guidance. This tendency is supported by both the NAEFS and ENS
Situational Ensemble Tables such that all levels rise above the 90th
and 97.5th percentiles on Tuesday and Wednesday respectively. CPC
guidance also shows a 60-70% chance of above normal temperatures for
the five day period of Mar 9-13th. Given both our seasonal
knowledge, and the strong probabilistic trends in guidance, high
temperatures were increased on Tuesday and Wednesday using a blend
of the GFS and NBM 90th percentile values. Low temperatures
overnight Tuesday into Wednesday were adjusted using the NBM 75th
percentile guidance.

There is a slight chance of precipitation on Tuesday as a shortwave
passes to our north, however given that this potential is five days
out, it is too soon to determine what localities, if any, will see a
brief passing shower. Otherwise, next week looks dry.

Guidance is narrowing in on a cold front/chance for more significant
precipitation sometime late next week. More attention will need to
be paid to this system as Thurs/Fri enter our forecast period as the
ECMWF, GFS and Canadian models all differ on exact timing.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

VFR today with northwest winds gusting to 25 kt. VFR tonight and
Saturday with winds diminishing...Saturday gusts will be near
20 kt.

KBOS TAF...High confidence in TAF. VFR.

KBDL TAF...High confidence in TAF. VFR.

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. Breezy.

Sunday through Monday: VFR.

&&

.MARINE...

Gusty northwest winds today with potential for 25 to 30 kt gusts.
Winds diminish later tonight and Saturday with wind gusts Saturday
around 20 kt. Seas 5 to 6 feet on the outer waters today, subsiding
to 5 foot seas on the southern outer waters Saturday.

Light freezing spray will a factor through Saturday. Favorable
conditions include air temps around freezing, sea temps 35F to
40F, and strong gusty northwest winds.

Gales will be dropped with this forecast. Expect Small Craft
Advisory conditions today and tonight, diminishing below that level
on Saturday.

.Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.

Sunday through Sunday Night: Winds less than 25 kt.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 9 PM EST this evening for
     ANZ230>237.
     Gale Warning until 4 AM EST early this morning for ANZ250-251-
     254.
     Small Craft Advisory until 7 AM EST Saturday for ANZ255-256.

&&

$$

SYNOPSIS...WTB/KS
NEAR TERM...WTB
SHORT TERM...WTB
LONG TERM...KS
AVIATION...WTB/KS
MARINE...WTB/KS

NWS BOX Office Area Forecast Discussion

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