BOX issues Area Forecast Discussion (AFD) at Jul 23, 3:01 PM EDT764
FXUS61 KBOX 231901
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
301 PM EDT Tue Jul 23 2024
.SYNOPSIS...
Cloudy and generally dry weather prevails this afternoon,
although a weak area of low pressure to spread rain showers
northward into southeastern New England tonight. A stronger
front will move across the region Thursday with more showers and
thunderstorms, followed by dry conditions Friday into early
next week with increasing heat by Sun and Mon
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
200 PM Update:
Quasi-stationary frontal boundary remains anchored south the
southern coastal waters, its position being reinforced
southward by the passage of a weak area of low pressure now
lifting NE from the waters NE of Nantucket. Although some
shallow mixing has taken placed and allowed from limited breaks
in overcast, weak ridge of high pressure extending southward
from ME/NH has helped maintain considerable cloudiness for most
of Southern New England. That's led to temperatures not moving
very far with current readings in the upper 60s to lower to mid
70s, and also has led to tempered instability values. So despite
elevated humidity levels, the lack of much sunshine has taken
the edge off that humidity.
Pop-up showers may still develop in interior MA, CT into
northwest RI in widely scattered coverage thru sundown, but the
muted instability (about 500-800 J/kg of surface based CAPE)
should mitigate the potential for thunderstorms.
Into tonight, modest cooling of the boundary layer likely to
lead to re-developing and expanding low cloud cover from SE to
NW. This occurs ahead of a weak area of low pressure now over
the TN Valley region, which is forecast to move NE toward the
lower Hudson Valley by early Wed AM. Associated showers tied to
this feature to overspread our southern coastal waters and
northward toward the Hartford-Providence-Plymouth corridor.
Looking overcast but generally dry elsewhere. Lows mid 60s to
around 70.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
3PM Update
Highlights:
* Periods of rain Wednesday morning will taper off late morning into
early afternoon. Muggy conditions with temperatures in the middle
and upper 70s.
* Spot shower and or thunderstorm possible during the afternoon and
early evening, mainly across western Massachusetts.
In general, Wednesday is expected to be fairly cloudy with continued
unsettled conditions, which will lead to below normal afternoon
temperatures across most of southern New England. The day's weather
is driven by cyclonic flow aloft and shortwave energy ejecting off
the Mid Atlantic coast and weak surface low moving northeast along a
quasi-stationary boundary. This renews a surge of showers generally
along the southern coast of Rhode Island and Massachusetts between
12z and 18z. The caveat, position of the boundary. Current thinking
it is extended south of Long Island and eastward to Nantucket, with
the heaviest rains at the immediate shore and over the southern
waters. But, if the boundary is further north so will be the heavier
rains. There remains a low threat for flash flooding given the
anomolously high PWATs and deep warm cloud layer. As result, the ERO
for Wednesday remains 'Marginal'.
Similar to Tuesday, showers ending by late morning/early afternoon
with a few breaks of sunshine. Perhaps a brief afternoon shower and
or thunderstorm, although there is little CAPE and a poorly forced
environment. There area with the best chance would be far western
Massachusetts, here forecast CAPE are between 500 and 1000 J/kg.
And, as with most cases, there will be poor mid-level lapse rates
and lack of effective shear, do not expect any organized strong to
severe thunderstorms.
Afternoon temperatures are cool, with many locations not getting out
of the 70s. The lower Connecticut River Valley would be the spot to
hit 80 degrees, essentially Hartford to Springfield. While it is
cool it will also be muggy/soupy with dew points in the low 70s.
Wednesday evening through the predawn hours of Thursday another hit
of rain is possible with a cold front. While not delivering any
relief from the humidity, more of a wind shift from the southeast to
the southwest. Overnight temperatures settle into the upper 60s and
low 70s with similar dew points. Given winds are light, there could
be areas of fog develop during the second-half of the night and
sticks around for the start of Thursday morning commute.
&&
.LONG TERM /THURSDAY THROUGH TUESDAY/...
3PM Update
Highlights...
* A few severe storms possible Thu afternoon/evening.
* Drier and seasonable conditions Fri into the weekend.
* Heat and humidity begins to return early next week.
Details...
Thu and Thu evening... Moist airmass with PWATs hovering around 2.0
inches will have been in place since yesterday by the time we reach
Thu. However, noting just a hair bit of mid-lvl drying coming out
of the mid-Canadian mid-lvl ridge that supports a slight uptick in
H7-H5 lapse rates to above 6.0 C/km during the afternoon and evening
as a relatively progressive cold front shifts SE across the region.
With this, models supporting ML CAPE values approaching 1000 j/kg
with peak heating and thanks to the approaching front, have most
ingredients to reinforce convective activity from W to E through the
afternoon/evening.
Regarding and severe potential...with these parameters in place,
combined with sfc dewpoints in the lower-mid 70s as noted by the
previous forecaster and modest low-mid lvl shear, would support some
disorganized cells. Tail end of current CAMs also support this at
this time. Noting low lvl v-profiles, so with the moist airmass in
place could be a wet downburst/damaging wind threat with convection
that gets going. This will be the most likely risk.
Otherwise, with the upper lvl trof still in the tilt process, the
front remains fairly progressive, and so most of the remaining
convection should be done by evening.
Fri into the weekend... Cold front will usher in a drier/cooler
airmass with originations from as far north as the Yukon. Noting
gradual ridge building to 1-2 std deviations by Sun, associated with
some fold-over ridging from the central US/Canada. Temps and
dewpoints closer to seasonal normals through the weekend.
Early next week... With the continued aforementioned ridging,
modified central US airmass combined with a bit of a feed of the
Gulf of Mexico developing, will see increasing heat and humidity
into the middle portion of next week. Typical model divergence with
mid-week shortwave likely fed by convective feedback issues as
guidance suggests MCS development upstream.
&&
.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18Z TAF Update:
Through 00z Wed: High confidence.
Most persistent IFR-LIFR cigs for the Cape and Islands, with
periods of mist and light NE winds at times. Elsewhere, mix of
generally VFR-MVFR ceilings; -SHRA develops for TAFs N/W of I-95
thru 00z Wed, some potl for TS BDL-BAF-ORH too, but is remote a
chance to include in TAFs. Light NE to E winds, trending N for
western airports, speeds under 5 kt.
Tonight and Wednesday: High confidence in trends, moderate on
timing.
IFR-LIFR conditions continue for the Cape and Islands tonight.
Mix of VFR-MVFR elsewhere should degrade to widespread MVFR
tonight from SE to NW, with periods IFR and mist developing
overnight. Mainly dry thru 04z, but risk increases for SHRA
after 04z mainly south of HFD-PVD-PYM line, which gradually
spreads northeast towards ORH and BOS by daybreak Wed. Timing of
the onset of widespread MVFR/IFR tonight and when (if?) it
starts to improve on Wed is uncertain; there could be some
optimism/1-category improvement by 14-16z Wed but that timing is
subject to adjustments. E to SE winds, speeds 4-7 kt.
Wednesday Night: Moderate confidence.
Deterioration likely Wed night into the MVFR-IFR range in all
areas on NE to N winds, though timing uncertain.
KBOS TAF... Moderate confidence in TAF. OVC VFR (FEW-SCT
015-025 bases lurking around) to continue until at least 02z.
Deterioration likely thereafter to MVFR after 04z, with possible
window of IFR after 08z. Unclear exactly when improvement may
occur but opted for 14z Wed for 1 category improvement. ENE/ESE
winds 4-7 kt thru the period.
KBDL TAF... Moderate confidence in TAF. OVC VFR bases with
possible SHRA thru 00z. Conditions then deteriorate to MVFR
overnight with possible IFR after 07z associated with light
SHRA and mist. Light N winds thru tonight, trending light SE
Wed.
Outlook /Thursday through Sunday/...
Wednesday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.
Thursday: Mainly VFR, with local MVFR possible. Chance SHRA,
isolated TSRA.
Thursday Night: VFR. Chance SHRA, isolated TSRA.
Friday through Saturday: VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Tonight through Wednesday Night... High Confidence.
Stationary boundary draped across the southern waters will provide a
source for shower and storms to develop tonight into the first-half
of Wednesday along with periods of poor visibility. Overnight and
Wednesday winds are from the ESE to SE with speeds 10 to 15 knots
and seas between 2 and 4 feet. Wind shift occurs late Wednesday
evening into Thursday morning as a weak front moves across the
waters, wind shift from SE to SW, wind speeds less than 10 knots
with seas 3 to 4 feet.
Outlook /Thursday through Sunday/...
Wednesday Night through Thursday: Winds less than 25 kt. Seas
locally approaching 5 ft. Chance of rain showers, isolated
thunderstorms.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, isolated
thunderstorms.
Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Friday Night through Saturday: Winds less than 25 kt.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
$$
SYNOPSIS...KJC/KS
NEAR TERM...Loconto/Hrencecin
SHORT TERM...Dooley
LONG TERM...MD
AVIATION...Loconto/Dooley/Hrencecin
MARINE...Dooley
Source:
BOX issues Area Forecast Discussion (AFD) at Jul 23, 3:01 PM EDT----------------
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