Lexington Weather

Lexington, Massachusetts, USA

61°F

5/20/2024 7:52pm

Lexington Conditions: Clear
Temperature: 61.0°F / 16.1°C
Dew Point: 54.1°F
Relative Humidity: 78%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 29.94 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for North Platte, NE

        
000
FXUS63 KLBF 181721
AFDLBF

Area Forecast Discussion
National Weather Service North Platte NE
1221 PM CDT Sat May 18 2024

.KEY MESSAGES...

- After a dry daytime Saturday, active weather including the
  threat for severe weather and heavy rainfall returns for
  Sunday through Tuesday.

- The threat for a late season frost appears limited but a brief
  cooldown around the middle of next week may support at least a
  localized potential.

- The extended forecast looks to remain active, but day-to-day
  predictability of precipitation chances remains low confidence
  at this time.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 422 AM CDT Sat May 18 2024

Quiet conditions are largely expected through the daytime Saturday. 
Currently, mid to high-level cloud cover continues to percolate 
across our southern and eastern zones as of 0800z (3am CDT). This is 
occurring in an area of modest mid-level lift via weak fgen ahead of 
a low-amplitude shortwave disturbance tracking through the northern 
Rockies. Though a few echoes are showing up on radar, no indication 
is apparent that any moisture is reaching the ground. This is likely 
due to aforementioned weak lift and surface dew point depressions of 
20 to 25 degF in the area. Lift will wane during the daytime hours 
as a progressive cool front dives south and lagging mid-level lift 
quickly follows. Northerly flow will settle across the forecast area 
which will help cool things a bit from Friday's well-above normal 
values. Leaned on NBM output plus short-term guidance for today's 
high temperatures. Even with appreciable CAA during the first half 
of the day, h85 temperatures will generally remain near 
climatological values. This will support daytime highs near 
climatological values too with afternoon values +/- 5 degF of 
average for mid/late May, ranging from upper 60s near the Pine Ridge 
to low 80s in far southwest Nebraska. Northwest winds will linger 
through the day until abruptly flipping to the southeast this 
evening as surface high pressure shifts east of the area. Areas of 
north central Nebraska may see some northwest gusts nearing 30 mph 
at times, but these are expected to decrease through the afternoon 
and fall to less than 15 mph by early evening.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 422 AM CDT Sat May 18 2024

Active weather returns tonight, lasting through the first half of 
the upcoming week. Rain and thunderstorms, with a threat for locally 
heavy rain and severe weather, are the pertinent messages for this 
time frame.

Beginning Saturday night, increasing southerly flow will settle 
across the central and southern High Plains as surface high pressure 
shifts east. With a strengthening LLJ, moisture flux will ramp up 
considerably. NAEFS guidance highlights vapor transport exceeding 
the 90th percentile for our western zones through the overnight 
hours into Sunday morning. This broad warm-air/moisture advection 
will be the focus for scattered rain and thunderstorms during the 
pre-dawn hours. With steep mid-level lapse rates across much of the 
region, instability will be plentiful for a mention of 
thunderstorms. Forecast soundings, however, show fairly meager 
instability aloft. MUCAPE values prior to dawn will largely be 
limited to less than 1000 j/kg outside of areas south of Interstate 
80. Fairly even distribution of the instability in a saturated 
profile does raise some concerns for locally heavy rainfall though. 
HREF ensemble max values show fairly widespread 0.75"+ QPF with 
pockets of 1.50-2.00". The question regarding this is how far north 
the focus for forcing makes it. SREF and NAM solutions depict this 
threat stretching from near MCK up through ONL whereas HREF guidance 
is further south closer to a GLD to EAR line. For now, leaned on 
SREF/NAM outputs for PoPs potential and the further north 
depictions. For the most part, this matches latest GEFS 
probabilities falls short of ECMWF and EPS guidance which both show 
a further north yet bullseye. Either way, areas that see the prime 
focus of lift will need to be mindful of heavy rainfall during the 
overnight hours.

Sunday...questions surround expanse of residual cloud cover and how 
that affects recovery ahead of the next round of rain and 
thunderstorms. Atmosphere recovery will be the key feature regarding 
the potential for severe thunderstorms during the afternoon and 
evening Sunday. Developing surface low across eastern Wyoming will 
occur in an area of enhanced southwesterly flow. At the surface, a 
dryline will extend south from the Black Hills through northeast 
Colorado and into far southwest Kansas. Continued southerly flow 
will usher in rich low-level moisture as surface dew points climb 
into the upper 50s to low 60s across a broad warm sector. Even with 
a broad warm sector and widespread moderate moisture quality, 
instability will be limited to a narrow corridor roughly between the 
Highway 385 and Highway 61 corridor. This is due to lingering cloud 
cover further east limiting insolation and keeping afternoon highs 
limited. Current forecast highs range from the middle 80s in 
southwest Nebraska to middle 70s across north central. This is the 
lingering uncertainty, predominantly regarding eastward extension of 
the greatest instability, limits confidence in precise details for 
Sunday convection. The latest SPC Day 2 Convective Outlook 
introduced a localized Slight Risk (level 2 of 5) across southwest 
Nebraska with the remainder of the local area in a Marginal Risk 
(level 1 of 5). Am in general agreement with this decision locally 
as unfavorable warm sectors have limited previous severe weather 
threats this year and believe this is shaping up to be another 
example of this. Thunderstorms should develop by late afternoon 
along the dryline as low-level convergence increases and capping 
erodes. Storm motion vectors will be northwest to southeast, which 
should allow residence time in the areas of greatest instability to 
last into the late evening hours. With h5 flow of 35 to 45 knots, 
sufficient deep-layer shear will exist for storm organization likely 
favoring discrete supercells with initial development as BWD vectors 
are largely orthogonal to the surface boundary. With MU-LI values 
nearing -10 degC and long/straight hodographs above 1km, believe 
hail to be the greatest threat until potential for upscale growth 
translates to a more damaging winds threat. Latest hi-res guidance 
shows a mature MCS diving east out of the higher terrain late Sunday 
evening into early Monday. This is likely due to merging cold pools 
and continued southerly flow feeding rich theta-e air into any 
ongoing activity. For now, believe this threat to be overplayed by 
guidance, notably the HRRR, but the potential will need monitored. 
Even so, rain and thunderstorm coverage will quickly track east and 
exit the area by early Monday morning. This will leave a lull of 
approximately 12 to 18 hours in the area before the next system 
arrives from the west.

Monday and Tuesday...amplifying flow will set up across the western 
CONUS on the southern periphery of an h5 low located in the lee of 
the Canadian Rockies. This feature will slowly drift east through 
Monday and into early Tuesday. As this occurs, southerly flow will 
continue to usher in richer moisture content through the profile. 
Both EPS/GEFS solutions show PWAT values exceeding 100% of normal by 
 early Monday with further increases through the daytime. The latest 
ECMWF Extreme Forecast Index (EFI) shows a broad area of anomalous 
QPF mainly for Monday night into Tuesday with non-zero Shift of 
Tails values. This suggests an anomalous rainfall event with non-
zero threat for some extreme values that may threaten some records. 
Height falls increase by Tuesday as the elongated trough ejects onto 
the Plains and a surface low lifts north and east across Kansas and 
into eastern Nebraska. Western Nebraska will fall squarely in the 
area of greatest deformation so lift will be fairly strong locally 
leading to the threat for rain and thunderstorms and potential for 
locally heavy QPF. Given these aforementioned ingredients...strong 
lift and anomalous moisture, the pieces are coming together for a 
notable precipitation event. NBM probabilistic guidance suggests a 
50/50 shot at seeing > 1.00" across central and northeast Nebraska. 
EPS/GEFS mean output for QPF on Tuesday range from 0.75" to 1.25" 
for much of the local area with deterministic solutions suggesting 
locally higher amounts. Latest WPC Excessive Rainfall Outlook 
includes a Marginal Risk for the area Monday. Believe this to be 
adequate at this time and localized flooding threat will need to be 
monitored. Temperatures will be held in check Monday due to 
expansive rain and clouds with values only reaching the 60s for much 
of the Sandhills and the low to middle 70s elsewhere. Cooler 
temperatures yet arrive Tuesday with highs only in the 50s and 60s 
for the whole of western Nebraska.

Wednesday and beyond...a convoluted pattern will set up by mid-week, 
lingering into the latter half of the week. The cutoff low across 
southern Canada will stall immediately north of North Dakota. 
Multiple disturbances will rotate around the southern periphery of 
this feature bringing recurring rain chances to much of the region. 
Generally speaking, day-to-day variability in temperatures won't be 
significant as 25th to 75th percentile ranges show minimal wavering 
for Thursday and beyond. So following one more cooler day on 
Wednesday, temperatures appear set to return to seasonable values to 
end the forecast period. Until the evolution of the main h5 cut-off 
becomes more certain, precipitation chances and when they'll be 
favored will remain fairly uncertain. What can be said though is the 
pattern appears to remain active and should support enough 
precipitation potential to stave off any prolonged dry spell at this 
time.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1221 PM CDT Sat May 18 2024

VFR will continue through this evening for western and north
central Nebraska terminals as high pressure provides fair skies
and a gradually waning northerly breeze. Gusts above 20kts
should decrease to below 12 kts by 18/21z. Northerly winds
transition to east and eventually southeast/south through the
course of the evening and nighttime. Clouds will also fill in
and lower as rain showers develop early tomorrow morning. MVFR
to locally IFR ceilings are likely for southern terminals (LBF)
after sunrise, while southerly winds also strengthen with gusts
exceeding 25 kts. 

&&

.LBF WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...NMJ
LONG TERM...NMJ
AVIATION...Snively