Flood Watch 1, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
333 PM EST Tue Dec 22 2020

.SYNOPSIS...
High pressure builds over the region Wednesday with sunshine and
seasonable temperatures. A strong storm system will bring heavy
rain, potentially damaging winds, and well above normal
temperatures Christmas Eve night into Christmas morning. Then,
dry and much colder conditions arrive this weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Blustery NW winds will continue into the evening in cold advection
pattern, then winds will gradually diminish overnight as high pres
builds in from the west and gradient relaxes. The exception will be
along the eastern MA coast and Cape/Islands where the gusty winds
will persist for much of the night. Diurnal clouds will erode this
evening with clearing skies during the night. Lows will be mostly in
the 20s with some upper teens across interior northern and western
MA.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday...

High pres builds south of New Eng with ridge axis extending across
the region. Large scale subsidence and dry air will promote mostly
sunny skies although expect increasing high clouds in the afternoon
as upper level ridge axis slides to the east. 925 mb temps similar
to today which should yield highs upper 30s to lower 40s.

Wednesday night...

High pres moves offshore with increasing clouds in developing warm
advection pattern. Some lower clouds may move into western New Eng
overnight. There is a window for radiational cooling in the evening
before clouds thicken and south winds develop. Lows will be mostly
in the mid/upper 20s, except lower 30s near the coast, but expect
rising temps after midnight, into the 30s with lower 40s near the
coast.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Powerful Storm to impact the region Christmas Eve night into
  Christmas morning with strong to damaging winds and heavy rain
  with minor flooding

* Colder but dry weather returns Christmas night into the weekend

Details...

*** High Wind Watches in effect for RI and Eastern MA Christmas Eve
night into Christmas Morning ***

High amplitude/full latitude trough taking on a negative tilt will
result in a threat for strong to damaging winds, heavy rain with
potential minor flooding, a fine line of low top convection and
minor coastal flooding. Will break things down by each hazard below.

Wind/Power Outages...

Anomalously strong low level jet crosses the region Christmas Eve
night into Christmas morning. Both ensemble systems NAEFS and ECMWF
have 850 mb and 1000 mb wind anomalies 5+ standard deviations along
with percentiles MAXED out! In addition, deterministic guidance
continues to trend upward with low level 925 mb jet up to 75-85 kt
of wind streaming across CT-RI and eastern MA Thu night into Fri
morning. While models do struggle with low level lapse rates on
southerly wind events, model time sections indicate 50-60 kt of wind
extending downward to below 1 kft! That's typically a good signal
for 50+ kt reaching the surface. Another rule of thumb, taking 60-
70% of 925 mb (75-85 kt) jet, which still yields 50+ to the surface.
Furthermore, model guidance advecting dew pts into the mid 50s ahead
of the front. This low level moisture and warmth will ease the low
level inversion and increase the wind threat. Even MOS guid for some
of coastal locations indicating 40+ kt of sustained wind! So in a
nutshell, ensembles, deterministic guidance and model soundings all
scream the potential for a high end wind event and residents need to
prepare for possible power outages Christmas Eve night into
Christmas morning. This combined with holiday approaching will issue
a 5th period High Wind Watch for RI and Eastern MA, this includes
Boston, Providence, Cape Cod and the Islands. After collaboration
with WFO ALY & OKX will hold off on interior CT given highest threat
is RI and Eastern MA as low level jet continues to increase as it
advects across those areas. However later shifts may have to expand
high wind watch westward into CT.

Heavy Rain/Flood Potential...

Anomalous moisture stream with Gulf of Mexico and Atlantic sources
yielding PWAT ensemble anomalies of 4+ standard deviations and IVT
+7! This combined with strong jet dynamics will yield a widespread 1-
2 inches of rain but potentially up to 2.5-3+ inches on the nose of
the low level jet across CT into RI and western-central MA with SSE
winds providing some upslope enhancement. This will likely yield
minor river flooding and obviously minor urban/street-highway
flooding. Given confidence is only low for moderate river flooding
and after collaboration with surrounding offices, will hold off on
flood watches at the moment. But later shifts will have to evaluate
trends especially if higher confidence for more widespread 2.5-3+
rainfall amounts, that would support moderate river flooding and
more significant urban/street-highway flooding.

Convection...

Strong signal for a fine line of low top convection along and ahead
of strong cold front late Christmas Eve night into Christmas
Morning. Strong thermal gradient with the front increases
confidence. Any low top convection will only enhance the severe
threat for strong/damaging winds aloft reaching the surface and
locally enhancing rainfall for isolated 3+ inch rainfall amounts.

Locally Dense fog...

With dew pts climbing thru the 50s across the snowpack, expecting
areas of locally dense fog possible Christmas Eve night into until
frontal passage Christmas morning.

Near Record Warmth...

Very strong low level thermal advection with dew pts rising to 55 to
perhaps 60 will support temps near 60 or perhaps low 60s. This will
approach record highs Christmas Day and smash record high min temps
Christmas Eve night.

Fri night into the Weekend...

Post frontal airmass will be blustery and colder, but close to
seasonal levels. Dry weather prevails.

Early next week...

Next chance of rain/snow showers Monday and/or Tue.

&&

.AVIATION /21Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Through 00z...
VFR. NW wind gusts to 25-30 kt, strongest interior with winds
increasing along the coast by late afternoon.

Tonight and Wednesday...High confidence.
VFR. Gusty winds in the evening then diminishing overnight, but
gusts 20-30 kt persisting overnight along the immediate coast
and Cape/Islands before diminishing during Wed.

Wednesday night...Moderate confidence.
Mainly VFR with increasing mid/high clouds, but patchy MVFR
cigs may develop overnight.

KBOS TAF...High confidence in the TAF.

KBDL TAF...High confidence in the TAF.

Outlook /Thursday through Sunday/...

Thursday: MVFR/IFR conditions possible. Windy with local gusts
up to 30 kt. Chance SHRA.

Thursday Night: Mainly MVFR, with areas IFR possible. Strong
winds with gusts up to 50 kt. SHRA.

Christmas Day: Mainly MVFR, with local IFR possible. Strong
winds with gusts up to 55 kt. SHRA.

Friday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SHSN.

Saturday through Saturday Night: VFR. Breezy.

Sunday: VFR.

&&

.MARINE...
Today and tonight...

Southeast swell from departing coastal low this morning will
maintain 5-6 ft seas on the outer waters today. Increasing
northwest wind late afternoon and night will bring gusts up to
25 kt, which will bring increasing seas on most of the waters.
Small Craft Advisory continues on most waters, and has been
expanded into Boston Harbor and Narragansett Bay for tonight.

Wednesday...

Small Craft Advisory conditions continue most waters into the
morning... and on the outer waters through sunset. Northwest
winds may linger early with gusts near 25 kt. But after mid-
morning, expect the headline to be supported only by rough seas.
Conditions continue to diminish through the afternoon/evening.

Outlook /Thursday through Sunday/...

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of rough seas. Slight chance of rain showers.

Thursday Night: Low risk for storm force winds with gusts up to
50 kt. Rough seas up to 18 ft. Chance of rain showers.

Christmas Day: Low risk for storm force winds with gusts up to
55 kt. Rough seas up to 18 ft. Rain showers likely. Areas of
visibility 1 to 3 nm.

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 14 ft. Chance of rain
showers.

Saturday: Winds less than 25 kt. Rough seas up to 11 ft.

Saturday Night: Winds less than 25 kt. Rough seas up to 9 ft.

Sunday: Winds less than 25 kt. Seas up to 5 ft.

&&

.CLIMATE...
Christmas Eve...

Boston Record High Minimum
1). 49F 2015
2). 44F 2014
3). 43F 1888
4). 42F 1964
5). 41F 1965

============================

Providence Record High Minimum
1). 50F 2015
2). 46F 1979
3). 44F 1965/1964
5). 42F 1987

============================

Worcester Record High Minimum
1). 47F 2015/1964
3). 43F 1979
4). 41F 1982
5). 38F 2014/1895

============================

Hartford Record High Minimum
1). 43F 2014
2). 42F 2015/1987
4). 41F 1979
5). 38F 1965/1964

Christmas Day...

Boston Record Highs
1). 65F 1889
2). 62F 2015
3). 60F 1964/1888
5). 59F 2014/1982/1915

Boston Record Precip.
1). 0.89" 1889
2). 0.82" 1886
3). 0.80" 1986
4). 0.79" 2002
5). 0.55" 1966

============================

Providence Record Highs
1). 64F 2015
2). 63F 2014/1964
4). 62F 1940
5). 60F 1965

Providence Record Precip.
1). 1.68" 1978
2). 1.19" 1986
3). 0.74" 2002
4). 0.63" 1974
5). 0.60" 1945

============================

Worcester Record Highs
1). 60F 2015/1964
3). 59F 2014/1940
5). 57F 1982

Worcester Record Precip.
1). 1.15" 1986
2). 1.05" 1965
3). 0.73" 1979
4). 0.58" 1979
5). 0.53" 1997

============================

Hartford Record Highs
1). 64F 1964
2). 60F 2015
3). 58F 1994
4). 56F 2014/1940/1915

Hartford Record Precip.
1). 1.39" 1979
2). 1.10" 1978
3). 1.08" 1986
4). 0.80" 1945
5). 0.79" 2005

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Wind Watch from Thursday evening through Friday afternoon
     for MAZ005>007-013>024.
RI...High Wind Watch from Thursday evening through Friday afternoon
     for RIZ001>008.
MARINE...Storm Watch from Thursday evening through Friday afternoon for
     ANZ230>237-250-251-254>256.
     Small Craft Advisory until 10 AM EST Wednesday for ANZ231>234-
     251.
     Small Craft Advisory until 7 AM EST Wednesday for ANZ230-236.
     Small Craft Advisory until 7 AM EST Wednesday for ANZ235-237.
     Small Craft Advisory until 4 PM EST Wednesday for ANZ250-
     254>256.

&&

SYNOPSIS...KJC/Nocera
NEAR TERM...KJC
SHORT TERM...KJC
LONG TERM...Nocera
AVIATION...KJC/Nocera
MARINE...KJC/Nocera
CLIMATE...

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