Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 211339
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
939 AM EDT Sun Apr 21 2024

.SYNOPSIS...
A ridge of high pressure will result in dry weather through 
Tuesday with mild days and chilly frost nights, along with 
elevated fire weather potential. Unsettled, wet and cool on 
Wednesday. Drier late in the week and heading into the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Forecast remains on track. Have just brought things in line 
with the latest observations. Mid and high clouds continue to
stream overhead. Otherwise, no significant impacts expected as
dry conditions continue.

330 AM Update...

* Dimming sunshine behind increasing mid level cloudiness today
* High temperatures mainly between 55 and 60 this afternoon

A wave of low pressure over the Carolinas early this morning 
will move east today and stay well south of our region. This 
will keep our weather dry today...although we do expect an 
abundance of mid- high level cloudiness to overspread the region
later this morning and afternoon. The mid level cloudiness will
result in dimming sunshine through the day. That being 
said...the airmass is quite dry which should allow for afternoon
high temperatures to reach mainly between 55 and 60. Westerly 
winds will gust to 20 to perhaps briefly 25 mph in spots by 
afternoon with good mixing.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Key Points...

* Clearing tonight...lows upper 20s to the middle 30s in many
  spots 
* Areas of frost expected to develop very late tonight
* Sunny on Monday with highs between 55 and 60

Details...

Tonight...

Low pressure well south of our region tonight will move further
east and further away from our area tonight. This will allow the 
mid-high level cloudiness to move offshore and result in skies
becoming mainly clear tonight. There is some westerly flow
aloft, but think winds will decouple in many of the low lying 
spots for a few hours overnight. This coupled with a very dry 
airmass in place will result in a good night of radiational 
cooling. Overnight low temps should drop into the upper 20s to 
the middle 30s across much of the region with the coldest of 
those readings in the typical low-lying spots of western MA. We 
did issue a Frost Advisory for CT/RI and southeast MA now that 
we are into late April.

Monday...

A reinforcing dry cold front will cross the region early Monday
morning. This will result in a dry west to northwest flow of 
air across the region. Model cross sections are quite dry, so 
expect sunny skies. Although 850T will be between -2C/-3C, we 
expect a dry super-adiabatic atmosphere on Monday with excellent
mixing. Afternoon high temperatures should recover between 55 
and 60 in most locations.

&&

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

* Strong radiational cooling Mon Night resulting in widespread 
  frost. 

* Mild and breezy on Tue with elevated fire weather concerns 
  possible. 

* Gusty winds and showers late Tue Night through Wed. Temps 
  trending cooler. 

* Windy, cool and dry on Thu. 

* Still dry, but trending warmer on Fri and Sat.

Monday Night...

A ridge axis builds from the TN Valley into the Mid
Atlantic/eastern Great Lakes Mon Night. The ridge builds 
offshore of Mid Atlantic by early Tue. High pressure builds 
overhead Mon Night, but shifts toward Nova Scotia by early Tue. 

Dry and quiet weather expected through this period with high 
pressure. This brings clear skies and light winds, which will 
result in strong radiational cooling. The NBM running way too 
warm for this setup, so went with the 10th percentile of 
guidance for lows. Should see temps ranging from the mid 20s to 
the mid 30s. Should see fairly widespread frost. Frost/freeze 
headlines will likely be needed in future updates. 

Tuesday...

Caught under cyclonic flow. A ridge axis will be just off the 
Mid Atlantic coast early on Tue, while a trough is over the 
Upper Mississippi River Valley. The ridge gets shunted further 
to the SE, while the trough digs into the central Great Lakes by
late Tue. High pressure nudges in from Nova Scotia, but will 
build further to the northeast. 

Another period of dry and quiet weather, but with gusty winds. 
Anticipate there will be elevated fire weather concerns given 
the well mixed boundary layer per NAM/GFS Bufkit soundings. 
Shows that we mix up to roughly 800 hPa and we should see some 
gusts around 20 to perhaps 25 mph at times during the afternoon.
In this type of setup guidance tends to under do the temps and 
we typically end up with drier dew points/RH values. Have a bit 
more uncertainty on the dew points/RH values as we will be under
southerly flow, which tends to keep our moisture up a bit more 
than say westerly flow. For now went with the 20th percentile of
guidance for the min RH, so values range from the mid 20s to 
the mid 40s. As for temps went with the 75th percentile of 
guidance, which results in highs ranging from the mid 50s to the
mid 60s.

Tuesday Night through Thursday...

Still stuck in cyclonic flow. The trough over the Great Lakes 
region digs into the eastern Great Lakes by early Wed. A 
northern stream trough digs from James Bay into Quebec. The 
trough may interact with the southern stream wave over New 
England on Wed, though there is uncertainty on how things 
evolve. The trough cuts off over northern New England late on 
Wed before lifting toward Newfoundland and Labrador on Thu. A 
frontal system slides in late Tue through Wed before moving 
offshore on Thu. Will be stuck between a high over the Great 
Lakes and a low over the Canadian Maritimes on Thu. 

As was the case yesterday the PWAT plume not appearing overly 
impressive with PWATs below 1 inch per EPS/GEFS and GEPS nil 
probs of values AOA 1 inch. Deterministic guidance showing we 
generally we be under roughly 0.75 to 0.85 inches. We do have a 
deeper SW/W low level jet at 850 hPa, which could help squeeze 
out the moisture available. At this point deterministic guidance
shows roughly 30-40 kt jet in place. Though there is 
uncertainty on the interaction between the two troughs. If they 
phase as some guidance indicates this would result in a strong 
jet, thus a higher wind potential especially as we turn into 
colder westerly flow. For now have just stuck with the NBM with 
some minor tweaks for precip timing. Should see roughly a 0.1 to
around 0.4 inches of precipitation. Though again a more intense
system could squeeze out a bit more moisture and could bring a 
few thunderstorms for portions of our region Wed.

High temperatures on Wed will be in the 50s. As for Thu we will
be cooler in wake of the system with highs in the low to mid 
50s. Could be quite windy due to a tightened pressure gradient. 
May dry out well enough that once again there could be elevated 
fire weather concerns, but this will hinge on how much precip is
realized on Wed.

Friday through Saturday...

Ridge axis builds into the Great Lakes region on Fri and into 
Sat, but a shortwave may ride the ridge as we head into late 
Sat. High pressure generally in control through this period, but
a frontal boundary may lift toward us late on Sat.

Dry and quiet weather anticipated with temperatures rebounding 
as we head into the weekend. Do have some uncertainty with the 
shortwave lifting toward us. Given it is spring time there could
be some convective influences. Have just stuck with the NBM for
now. Highs around seasonable levels on Fri and slightly warmer 
than normal on Sat.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

Today...High confidence.

VFR with increasing mid level clouds. W winds 10 to 15 knots 
with gusts into the lower 20 knots at times this afternoon at 
some locations. 

Tonight and Monday...High confidence.

VFR conditions continue tonight and Monday. WNW winds generally 
5-10 knots. Some risk that a weak sea breeze will develop late 
Monday afternoon along portions of the eastern MA coast, but 
that is still uncertain at this time.

KBOS TAF...High Confidence in TAF. 

KBDL TAF...High Confidence in TAF. 

Outlook /Monday Night through Thursday/... 

Monday Night: VFR. Patchy frost.

Tuesday: VFR. Breezy. 

Tuesday Night: VFR. Breezy. Chance SHRA.

Wednesday: Mainly VFR, with local IFR possible. Breezy. Chance 
SHRA.

Wednesday Night: VFR. Breezy. Chance SHRA.

Thursday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Monday...High Confidence.

A relatively weak pressure gradient will keep winds and seas
below Small Craft Advisory thresholds today through Monday. That 
being said, there is enough heating over the land today to
result in westerly near shore wind gusts into the lower 20 knots 
for a few hours this afternoon.

Outlook /Monday Night through Thursday/... 

Monday Night through Tuesday: Winds less than 25 kt. 

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

Wednesday through Wednesday Night: Moderate risk for Small
Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.
Chance of rain showers. 

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

&&

.FIRE WEATHER...
Today and Monday...

There will be elevated fire weather concerns today, and again 
on Monday. Given that we are in a pre-greenup, dry westerly 
flow will allow for minimum afternoon relative humidity values 
to drop to between 20 and 30 percent both days, with Monday
being the slightly drier day. Westerly winds will gust to 
around 20 mph, to perhaps briefly 25 mph, in some spots this 
afternoon. The winds will be tad lower on Monday, but still 
expect WNW gusts of up to 20 mph.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Frost Advisory from midnight tonight to 7 AM EDT Monday for 
     CTZ002>004.
MA...Frost Advisory from midnight tonight to 7 AM EDT Monday for 
     MAZ013-016>023.
RI...Frost Advisory from midnight tonight to 7 AM EDT Monday for 
     RIZ001>008.
MARINE...None.

&&

$$

SYNOPSIS...Frank/BL
NEAR TERM...Belk/Frank/BL
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL
FIRE WEATHER...Frank