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Area forecast discussion 
National Weather Service Boston/Norton Massachusetts 
735 PM EDT sun Oct 21 2018 


Synopsis... 
high pressure ridge will build across the region tonight 
through Monday, with dry conditions and cold temperatures. A 
weather system crosses New England late Monday night and 
Tuesday with showers. Colder air then returns Tuesday night 
through Friday. A coastal storm may bring rain to the region 
Saturday night and Sunday. 


&& 


Near term /until 6 am Monday morning/... 
as skies become mostly clear and winds continue to diminish 
tonight, expect a rather cold, frosty night. As the airmass 
dries out, dewpoints will fall to the upper teens and 20s, which 
will also allow temps to fall quickly, mainly around or after 
midnight. Expect readings by daybreak to bottom out mainly in 
the 20s. The only locations that will remain close to or above 
freezing will be along the immediate coastline. 


With these conditions overnight, a freeze warning is in effect 
for the immediate eastern coast of Essex, Suffolk, and Norfolk 
counties, as well as southern Plymouth and Newport County. 


Low level clouds currently over upstate New York are starting to 
slowly move southeast into western Massachusetts. However, these clouds will 
likely dissipate on the eastern slopes of the Berkshires, having 
minimal impact on the Connecticut River valley. 


&& 


Short term /6 am Monday morning through Monday night/... 
Monday... 
high pressure ridge will build across the region during Monday, 
with the large center across the southeast U.S. 500 mb heights will slowly 
rise during the day, but even with a lot of sunshine, temps 
will remain on the cool side for late October. Temps should top 
off in the 50s, except only the mid-upper 40s across the east 
slopes of the Berkshires and Worcester Hills. Expect light west 
winds to begin to back to SW during the day. High clouds may 
also begin to filter across the region from midday through the 
afternoon. 


Monday night... 


12z model suite all signaling some mid level short wave energy 
working quickly east-southeast out of the Great Lakes in the fast flow 
aloft. However, timing the arrival of this energy is in 
question. Should see increasing clouds Monday night as the short 
wave's surface reflection works toward the eastern Great Lakes 
after midnight. 


Guidance suggests that some convergence tries to trigger some 
shower development along S coastal areas toward midnight, but 
there is not a lot of moisture to work with. At this Point, May 
see scattered showers develop after midnight across Cape Cod and 
the islands, with a few spotty showers further inland toward 
daybreak. If temps are cold enough across north central and northwest mass, 
and the precip does develop, could see some mixed rain/snow 
showers. Overnight lows will fall to the lower-mid 30s well 
inland, ranging to around 30 at the coast. 


&& 


Long term /Tuesday through Sunday/... 
big picture... 


Pattern starts the long-term period with a trough over the eastern 
USA, a ridge over the west, and a closed low over the Gulf of 
Alaska. The land features show some progressivity with the eastern 
trough moving across New England Wednesday and slowly lifting 
northeast through the Maritimes. Shortwave ejects from the Gulf of 
Alaska low and follows the northern jet stream across Canada 
midweek, then digs south as it phases with a southern stream 
shortwave from the Desert Southwest. This phasing creates a deep 
trough over the eastern USA next weekend and turns the general flow 
up the East Coast to New England. 


Contour heights are below normal through the week, and near normal 
over the weekend. This suggests a colder than normal week, 
moderating over the weekend. 


Mass fields are in general agreement nationally through Wednesday, 
and over New England through Thursday. There are differences with 
the handling of the northwest shortwave and its phasing with the 
southern shortwave. All models show the trough developing over the 
southeast USA this weekend, suggesting a developing coastal storm, 
but with differences that lead to different potential storm tracks. 
Confidence is high through Thursday, and low-moderate over the 
weekend. 


Concerns... 


Tuesday... 


Upper trough approaches from the Great Lakes, supported by a 105 kt 
upper jet that is aimed at the New England South Coast. At the 
surface a low pressure area over the St Lawrence Valley will swing 
both a warm front and then a cold front across our area. Expected 
placement of the nose of the upper jet would support coastal surface 
low developing either over or just south of southern New England. 
The moisture overhead is limited to 600 mb and lower, but this same 
layer shows lift through the layer, especially along the coast. We 
will continue to show chance pops for most of our area for this 
event. 850 mb temps of 1-3 suggest Max temps in the lower 60s, but 
clouds much of the day will work against mixing and so Max temps 
should be a little lower...we went with 50s. 


The system moves off through the Maritimes Tuesday night. Sfc 
pressure changes are forecast light but positive and should tighten 
the gradient enough for a stiff northwest wind. 


Wednesday through Friday... 


Upper trough overhead Wednesday with cold pool temps as low as -32c. 
This will support mixing to 850 mb Wednesday and Thursday. Northwest 
winds aloft in the column reach 25 kt each day, so expect gusts to 
that speed. Temps aloft at -3c to -5c Wednesday suggests Max temps 
in the lower 50s, and -7c to -9c Thursday suggests Max temps in the 
40s...the higher terrain near 40. Moisture remains limited each day, 
but enough for clouds and widely scattered showers. Nighttime temps 
will trend colder each night as winds trend lighter. Expect temps 
around freezing Wednesday night and mid 20s to low 30s Thursday 
night. 


High pressure moves overhead Friday. This will mean light winds and 
clearer skies. Temps aloft suggest Max sfc temps in the 40s. 


Saturday-Sunday... 


Different models, different forecasts. The GFS shows high pressure 
offshore but in control through the day Sat with increasing clouds 
but dry weather, then rain at night. The European model (ecmwf) shows increasing 
clouds with rain CT-RI-se mass Sat afternoon. The ggem shows dry 
Saturday and rain Sunday. Only confidence is that there will be wet 
weather sometime over the weekend, high uncertainty/low 
confidence...take your pick...as to the details. 


&& 


Aviation /00z Monday through Friday/... 
forecaster confidence levels... 


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


Short term /through Monday night/...high confidence. 


Tonight... VFR. Winds diminish away from the coast this 
evening, but will linger through around 06z or so along the 
coast before diminishing. West-northwest winds slowly shift to SW toward 
daybreak. 


Monday and Monday night... 
VFR conditions. SW winds 5-10 kts. Clouds increase with low 
chance of spotty showers especially along the S coast after 
midnight. 


Kbos terminal...high confidence in taf. 


Kbdl terminal...high confidence in taf. 


Outlook /Tuesday through Friday/... 


Tuesday: mainly VFR, with local MVFR possible. Breezy. Chance 
rain showers. 


Tuesday night through wednesday: VFR. Breezy. Slight chance 
rain showers. 


Wednesday night through Thursday night: VFR. Breezy. 


Friday: VFR. 


&& 


Marine... 
forecaster confidence levels... 


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


Short term /through Monday night/...high confidence. 


Through 00z... 
leftover gale warnings through 00z on the southern outer 
waters, otherwise small craft advisories in place. 


Tonight... 
high pres S of the waters will keep west-northwest winds in place. 
Leftover gusts up to around 25 kt through most of the night 
across the open waters. 


Monday and Monday night... 
west winds gusting up to 25 kt early Monday on the eastern waters, 
then should drop below small craft levels by around midday. 
Winds shift to SW by Monday evening, with some gusts to 20 kt 
possible. Some visibility restrictions possible in spotty 
showers mainly around or after midnight Monday night. 


Outlook /Tuesday through Friday/... 


Tuesday: low risk for Small Craft Advisory winds with gusts up 
to 30 kt. Local rough seas. Chance of rain showers. 


Tuesday night: moderate risk for Small Craft Advisory winds 
with gusts up to 25 kt. Local rough seas. Slight chance of rain 
showers. 


Wednesday: moderate risk for Small Craft Advisory winds with 
gusts up to 25 kt. Areas of seas approaching 5 ft. Slight 
chance of rain showers. 


Wednesday night: moderate risk for Small Craft Advisory winds 
with gusts up to 30 kt. Areas of seas approaching 5 ft. Slight 
chance of rain showers. 


Thursday: low risk for Small Craft Advisory winds with gusts up 
to 25 kt. Local rough seas. Slight chance of rain showers. 


Thursday night: low risk for Small Craft Advisory winds with 
gusts up to 25 kt. Local rough seas. 


Friday: winds less than 25 kt. Seas locally approaching 5 ft. 


&& 


Box watches/warnings/advisories... 
CT...none. 
Massachusetts...freeze warning from 11 PM this evening to 9 am EDT Monday for 
maz007-015-016-021. 
Rhode Island...freeze warning from 11 PM this evening to 9 am EDT Monday for 
riz005-007. 
Marine...Gale Warning until 10 PM EDT this evening for anz231-232-251. 
Small Craft Advisory until 3 am EDT Monday for anz233-234. 
Small Craft Advisory until midnight EDT tonight for anz230. 
Small Craft Advisory until 10 PM EDT this evening for anz236. 
Small Craft Advisory until 2 am EDT Monday for anz235-237. 
Gale Warning until midnight EDT tonight for anz250. 
Gale Warning until 11 PM EDT this evening for anz254>256. 


&& 


$$ 
Synopsis...wtb/evt 



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