BOX issues Area Forecast Discussion (AFD) at Dec 11, 2:45 PM EST684
FXUS61 KBOX 111945
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
245 PM EST Wed Dec 11 2024
.SYNOPSIS...
A strong cold front approaching from the west will bring periods of
heavy rain into this evening along with a round of strong to
damaging wind gusts later today into this evening. The heavy rain
may also result in some stream/mainstem river flooding along with
the risk for localized urban flooding. Dry, blustery and seasonable
temperatures on tap for Thursday behind this system. Below normal
temperatures and dry conditions expected Friday and Saturday.
Sunday through early next week trends more unsettled with
periods of precipitation possible with normal to slightly above
normal temperatures.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
Update: 10 AM
The warm front is lifting north across southern New England with
a vast difference in temperatures on either side of the front.
An example, earlier this morning the front had reached KTAN
where the temperature was 59F and at our office in Norton (only
8.5 miles away) out temperature was 43F. As the warm front
lifts further north and warms the low levels of the boundary
layer we will start to have stronger wind gusts mix down to the
surface and as of writing this update winds are now gusting (in
the area of the warm sector) between 20 and 30 knots.
Lastly, we are monitoring the potential for nuisance/minor
coastal flooding during todays high tide (4:17PM) for areas along
Narragansett Bay. Right now, the peak surge and strongest winds
appear to arrive between 1-2 hours after high tide, but the
persistent wind up Narragansett Bay could limit the amount of
evacuation of the water and may lead to MLLW of 7.0' to 7.5',
flood stage of Fox Point is 7.0'. This is something we will
continue to monitor.
Key Points...
* Widespread 2-3" of rain with localized 4"+ amounts possible
* Some stream/mainstem river and localized urban flooding possible
* Strong to Damaging Wind Gusts of 50-60 mph later today into tonight
* A few embedded t-storms or a fine line may enhance the wind threat
Details...
Today into this evening...
A rapidly intensifying low pressure system developing in the mid
Atlantic States will track northward into PA/NY today and into
northern New England/Quebec areas by early evening. This is all
being driven by a vigorous trough to our west. The result will be a
period of strong to damaging wind gusts, periods of heavy rain along
with the potential for some flooding and perhaps a few embedded
thunderstorms. We will break it down more below.
1) Heavy Rain and Potential Flooding:
Deep meridional flow ahead of a vigorous trough and rapidly
intensifying low pressure system will bring periods of heavy rain
today into this evening. Warm advection aloft over the remaining
cool dome at the surface was enhancing the forcing for
ascent...resulting in rain developing from the west very early this
morning. In fact...a lead shortwave coupled with an approaching warm
front will bring a period of widespread rain for a portion of the
morning. Pwats around 1.25 inches coupled with forcing along the
warm front will bring periods of heavy rain.
We then will probably see a partial lull in the activity towards
lunchtime. This a result of the better forcing passing to our north
with the lead shortwave/warm front. However...still expect scattered
showers at times given the strong warm advection pattern and high
PWATs in place.
As the cold front approaches...the second round of widespread heavy
rain will overspread the region from west to east later this
afternoon and into this evening. Pwats rise to around 1.50 inches
and this will be accompanied by very strong forcing with a southerly
LLJ of 80-90+ knots developing at 850 mb. Given the combination of
high PWATS and very strong forcing...a windswept heavy rain will
overspread the region from west to east later this afternoon and
evening. A widespread 2-3" of rain is expected with localized 4"+
amounts not out of the question based on the latest HREF and some of
our other CAMs. A lot of this heavy rain may coincide with the
evening commute and result in the potential for localized urban
flooding. This threat would be enhanced if we see a fine line
develop with or without lightning...which we will discuss more in
another section. We may also see some stream/mainstem River Flooding
exacerbated by snowmelt in northern New England. We opted to expand
the Flood Watch into central MA, RI and portions of southeast MA.
2) Strong to Damaging Wind Gusts:
The greatest concern revolves around the potential for a period of
strong to damaging wind gusts...especially across eastern MA and RI
later today into this evening. As we talked about in the prior
paragraph...a powerful southerly LLJ of 80-90+ knots at 850 mb will
develop. Current temps in the 30s to the lower 40s early this
morning will rise sharply to near or above 60 degrees later today
into this evening. This process will occur quickly across RI/SE MA
later this morning and early afternoon. However...the HRRR/NAM which
usually handle shallow cold air the best indicate that it may take
until late afternoon to scour our the remaining shallow cold air
across parts of the CT River Valley and parts of interior northern
MA.
Nonetheless...we do expect the warm sector to have advanced across
the entire region by late afternoon/early evening. Temps rising to
near or above 60 degrees will allow for the strong winds to mix
down. It is always very difficult to determine how much wind is able
to mix down in these events...which is often determined by
mesoscale. One window of opportunity for strong to damaging winds of
50-60 mph will occur later today...where a lull in the rain may
allow temps to rise a bit more and promote better mixing. Another
opportunity will be along a potential fine line with or without
lightning late this afternoon into this evening. We will discuss
this convective threat in more detail in the next section. Given the
strong winds will be accompanied by heavy rain...the risk for tree
damage and power outages will exist.
We opted to extend the High Wind Warning into the rest of eastern MA
and RI based on the above reasoning. In fact...there is at least a
low risk that portions of the advisory may approach high wind
criteria depending on some mesoscale factors especially with a
potential fine line. Regardless...tree damage and power outages are
a concern.
3) Convective/Fine Line Threat...
Lastly...the model guidance is indicating a few hundred J/KG of
MUCape ahead of the approaching cold front and the powerful
southerly LLJ late today into this evening. Whether or not this is
accompanied by embedded t-storms remains uncertain...but it will
bring the potential to enhance the strong wind risk with the
powerful winds just of the deck. SPC has put the southern portions
of our area in a marginal/slight risk. This is going to be more of a
nowcast situation...but will have to monitor closely.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
Key Points...
* Rain/strongest winds should exit the coast after midnight
* Dry & blustery Thu with highs upper 30s to middle 40s
After Midnight...
The bulk of the heavy rain/strong winds should exit the coast not
too long after midnight. A drier westerly flow of cooler air will
begin to work back into the region. Rain may end as a brief period
of snow or flurries along the east slopes of the Berks...but even
out there not expecting a real impact. Overnight low temps should
bottom out in the 30s to the lower 40s. It will remain breezy as the
cooler air works back into the region...but nothing like we are
expecting later today into this evening.
Thursday...
A mixture of clouds and sunshine under modest cold advection will
result in a much chillier day. High temps will be in the upper 30s
to the middle 40s. It will remain rather breezy too with gusts of 25
to 35 mph expected.
&&
.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Key Messages:
* Below normal temperatures favored Friday and Saturday
* Dry conditions through the first half of weekend. Next chances for
precipitation Sunday through early next week.
High pressure continues to build in from the south Thursday night,
promoting dry conditions and minimal cloud cover. The deeper cold
air shifts over the region and with efficient radiational cooling
will owe to a fairly chilly night. Temperatures drop into the upper
teens to low 20s across the interior with mid teens across the
higher elevation spots. Coastal areas range in the low to mid 20s.
Ensembles show the coolest/anomalous 850mb temperatures shifted over
the northeast by Friday bringing likely the coolest day of the week.
Highs will likely be below normal for this time of year in the 30s.
With high pressure in place, conditions remain dry. Expect another
cool night again Friday night with temperatures dropping into the
low to mid teens for the higher elevation interior, upper teens for
the rest of the interior, and low to mid 20s on/near the coast.
Temperatures slowly begin to moderate slowly Saturday and more
Sunday as the cooler airmass shifts away. Temperatures will still
stay below normal Saturday and highs Sunday will likely range closer
to normal in the upper 30s/40s.
The next chance for precipitation comes Sunday into early next week.
Ensemble guidance shows a general signal for increased moisture
phased with the passage of a few shortwaves in the flow aloft.
Confidence in the details remain low with individual solutions
struggling to agree on timing and strength of these waves. Overall
not seeing a substantial signal for heavy rainfall and more likely a
an unsettled pattern accompanied by periods increased rain chances
Sun-early next week. Temperatures trend warmer during this period
with temperatures normal to slightly above normal.
&&
.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18z TAF Update:
Through 06z Tonight...Moderate confidence.
Dynamic system brings periods of moderate to heavy rainfall,
strong wind gust, and low ceilings and visibility through
roughly midnight. Persistent LIFR to IFR conditions will
continue with improvement from west to east after 06z. As for
the wind, gusts have begun to increase due to mixing of the
boundary layer with the arrival of warmer surface temperatures,
with the strongest winds still to come, really increasing after
20z this afternoon with strongest winds around 23z, with gusts
40-50 knots across all terminals. In addition, the LLWS is
significant, with the LLJ at 2,000 between 70-80 knots. As the
previous forecast mentioned, a fine line could develop this
afternoon with a low chance of thunder.
06z Tonight into Thursday...High Confidence.
The bulk of the rain and wind will exit the coast not too long
after 06z. Otherwise, conditions should improve to mainly VFR
toward 08z west and 11z east. Winds shift to the west, gusts of
25-35 knots continues through Thursday afternoon.
Thursday Night...High confidence.
VFR. Diminishing west wind, gust less than 20 knots.
KBOS Terminal...Moderate Confidence in TAF.
KBDL Terminal...Moderate Confidence in TAF.
Outlook /Thursday Night through Monday/...
Thursday Night through Friday: VFR. Breezy.
Friday Night through Saturday Night: VFR.
Sunday: VFR. Slight chance RA, slight chance SN.
Sunday Night: Mainly VFR, with local MVFR possible. Chance RA,
chance SN.
Monday: Mainly VFR, with local IFR possible. Chance RA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
245 PM Update:
Key Messages:
* Dangerous Conditions for Mariners
* Southerly Storm-Force Gusts to 55 kt Late This Afternoon through
Early Overnight, Seas Offshore Building up to 18 Ft.
* Southwest to West Gusts 35-40 kt Early Thurs AM, Gradually
Decreasing Thru Thurs.
Late This Afternoon through Overnight: High confidence.
Dangerous conditons are expected for mariners in this period, with
southerly wind gusts increasing significantly to 45-55 kt starting
late this afternoon, peaking thru midnight. Very rough waves with a
deep southerly fetch, with wave heights peaking up to 18 ft
offshore, with 6 to 10 ft waves in the nearshore non-bay areas. A
windshift to WSW to W is expected overnight with gusts in the 35-45
kt range. Storm Warnings are in effect for all waters and those will
need to be downshifted to Gale Warnings as conditions warrant, with
Gale conditions on all waters extending into a part of Thursday.
A line of visibility-reducing downpours will also move through the
waters tonight, potentially bringing the risk for lightning as well
as a low probability for waterspouts.
Thursday: High confidence.
W gusts 30-35 kt thru Thurs, although gusts will be gradually
subsiding through the aftn hours to SCA levels (25-30 kt) by late
aftn/sundown. Gradually decreasing seas to around 10 ft over outer
waters by sundown, and around 3-5 ft nearshore (non-bays).
Thursday Night: High confidence.
Westerly gusts decrease to around 20-25 kt early Thurs night, then
becoming around 15-20 kt into early Fri AM. Gradually decreasing
seas to around 3-6 ft, with a continued need for SCAs into the
evening/overnight.
Outlook /Friday through Monday/...
Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft.
Saturday through Sunday: Winds less than 25 kt.
Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain.
Monday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through Thursday morning for CTZ002>004.
Wind Advisory until 1 AM EST Thursday for CTZ002>004.
MA...Flood Watch through Thursday morning for MAZ002>004-008>012-
017>021-026.
Wind Advisory until 1 AM EST Thursday for MAZ004-010>012-026.
High Wind Warning until 1 AM EST Thursday for MAZ005>007-
013>024.
RI...Flood Watch through Thursday morning for RIZ001>007.
High Wind Warning until 1 AM EST Thursday for RIZ001>008.
MARINE...Storm Warning from 7 PM this evening to 1 AM EST Thursday for
ANZ230.
Storm Warning until 1 AM EST Thursday for ANZ231>234-250-251-
254>256.
Storm Warning until 11 PM EST this evening for ANZ235>237.
&&
$$
SYNOPSIS...Frank/Mensch
NEAR TERM...Frank/Dooley
SHORT TERM...Frank
LONG TERM...Mensch
AVIATION...Frank/Dooley
MARINE...Loconto/Dooley/KP
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 11, 2:45 PM EST----------------
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