Author Topic: [Alert]IWX issues Area Forecast Discussion (AFD) at May 19, 5:47 PM EDT  (Read 363 times)

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IWX issues Area Forecast Discussion (AFD) at May 19, 5:47 PM EDT

359 
FXUS63 KIWX 192147
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
547 PM EDT Mon May 19 2025

.KEY MESSAGES...

- Rain chances increase from southwest to northeast late tonight
  into Tuesday morning.

- Rainfall amounts Tuesday-Tuesday night may range from greater
  than 1 inch across portions of west central and northwest
  Indiana to around one quarter of an inch south central Lower
  Michigan.

- Widespread rain with a few embedded thunderstorms is expected on
  Tuesday. Severe weather is not anticipated.

- Cool weather with periodic chances for rain continues during
  the second half of the week.

&&

.DISCUSSION...
Issued at 402 PM EDT Mon May 19 2025

In the near term, some rain showers have developed across
northern Illinois this afternoon in association with remnant
upper level small scale disturbances from last night's upstream
Plains convection, but dry downstream airmass should not allow
any measurable rain to penetrate much into local forecast area
this evening.

A stable longwave pattern is in place across most of CONUS this
afternoon with blocking characteristics across northeast
CONUS/southeast Canada. The dominant upper level trough resides
from Northern Plains to the Southern Plains. A mid/upper speed
max diving through upstream portion of this trough will continue
to intensity upper vort max at the base of this trough across
the Southern Plains this evening. The northeast ejection of this
feature will likely be tied to the majority of the forcing
helping to shift low level thermal/moisture gradient back to the
north across the southern Great Lakes on Tuesday with
widespread rain affecting the area. These showers and perhaps
isolated storms should work into far southwest portions of the
forecast area in the 06Z-12Z timeframe. Still some uncertainty
persists regarding the onset time of these rain chances as back-
door like cool frontal feature shifting from eastern Great
lakes to southern Great Lakes will tighten the low level
moisture gradient across the region overnight. This evolution
looks to continue to delay onset of PoPs across south central
Lower Michigan/northwest Ohio during the day Tuesday.

Rain becomes widespread for a time on Tuesday as strong low-mid
level theta-e boundary lifts northward with an accompanying
period of strong low level moisture transport, particularly
central/southern sections of the forecast area. Severe weather
is still not expected as mid level lapse rates will remain on
the weak side Tuesday, and potential of surface based convection
should remain south of the local area. By mid-late afternoon,
should start to see some decreasing rain shower coverage from
southwest-northeast as the initial southern Plains vort max
lifts northeast taking stronger isentropic upglide
northeast/east of the area. Rainfall amounts Tuesday-Tuesday
night may total an inch or more across the southwest but will
likely taper to a quarter of an inch or less across the far
northeast as better moisture return becomes more limited with
northward extent. Otherwise, Tuesday will feature windy
conditions as the combination of suppressed nature of surface
low pressure and blocking pattern allows maintenance of high
background pressure to the north acting to keep a strong low
level height gradient in place. Some easterly gusts to 30+ mph
are expected during the day Tuesday.

The active pattern will continue Tuesday night into Wednesday. The
blocking nature of this pattern appears to allow an elongated
north-south oriented upper PV filament to cut-off across
southern Ontario on Wednesday. To the south of this developing
negative upper height anomaly, additional eastern Pacific highly
sheared disturbances will allow for continued upper level short
wave troughs to impact the southern Great Lakes and Ohio
Valley. The more robust of these Pacific waves should move into
the Mid MS Valley later Tuesday night and will be accompanied by
some steeper mid level lapse rates and renewed low level
moisture transport. This could yield some scattered to numerous
showers/isolated thunderstorms with uptick in weak elevated
instability expected. Severe weather is not expected, but may
not be able to completely rule out some small hail across the
south for a brief time window Tuesday night.

Scattered showers and cooler than normal temperatures will
characterize Wednesday through Friday as a consolidated broad upper
level negative upper height anomaly persists across the region. Rain
showers during this period will bring only modest additional
rainfall amounts. By next weekend, mid/upper level height rises
suggest precip chances will be more limited which supports blended
PoPs in the 15-25 percent range periodically in the Sat-Mon period.
This evolution should also support temperatures closer to normal,
but likely remaining just below normal for late May.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 530 PM EDT Mon May 19 2025

Dry northeast to east flow at the surface at the start of the
TAF period will increase from the east and become gusty as the
next system nears. Rain will slowly saturate the lower layers
with lower ceilings lagging after the start of the
precipitation. Brought in MVFR between 15Z and 17Z at both
sites. Brief IFR is possible, but for now kept it out.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory from midnight tonight to 2 AM EDT
     Wednesday for LMZ043-046.

&&

$$

DISCUSSION...Skipper
AVIATION...Skipper

Source: IWX issues Area Forecast Discussion (AFD) at May 19, 5:47 PM EDT

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