Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 211116
AFDBOX

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

.SYNOPSIS...

A ridge of high pressure will result in dry weather today 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/...

705 AM Update...

Forecast remains on track. Have just brought things in line with
the latest observations.

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 /11Z 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 in 
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/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 and 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 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...Frank/BL
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL
FIRE WEATHER...Frank