Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue May 17 2022
Valid 12Z Fri May 20 2022 - 12Z Tue May 24 2022
...Persistent record-breaking heat is likely in the south-central
U.S. through this week with anomalous heat expanding into the East
this weekend...
...Heavy rainfall and flash flooding possible near a strong
northern tier system on Thursday...
...Overview...
The pattern across the lower 48 will amplify again by the start of
the medium range period on Friday with a digging trough over the
West/north-central U.S. and rising heights in the East. This
yields cooler temperatures and lowering snow levels in the West
and a continued hot regime for the South and East late this week.
A deepening low pressure system tracking from the northern
tier/Upper Great Lakes into Canada will push a strong cold front
eastward, with increasing chances for heavy rainfall and severe
weather along and ahead of it. The front/trough will eventually
reach the East this weekend as the guidance continues to be very
uncertain with details upstream across the Northwest into early
next week. A northward surge of tropical moisture will also
increase rain/thunderstorm chances across Florida late this week
and into the weekend.
...Guidance Evaluation/Preferences...
Overall, guidance continues to show a more open shortwave across
the northern Plains/upper Mississippi Valley on Friday, although
the 00z UKMET came in showing a compact closed low again, so some
uncertainty remains with that particular feature. Regardless, the
models show good agreement (with some minor timing differences) on
amplified troughing sliding into the East by the weekend. After
Saturday/day 4, the most prominent source of uncertainty continues
to be flow evolution across the West. The GFS and many GEFS
members continue to be much stronger/more pronounced with energy
on the backside of the trailing trough into the Northwest this
weekend, which eventually results in another fairly amplified
shortwave into the Intermountain West/Rockies by early next week.
On the other extreme, recent runs of the ECMWF are rather
flat/more ridgy with the flow over the West late period. This all
seems to stem from differences in how quickly energy from the
North Pacific moves into western North America. The ECMWF is much
faster to break down the East Pacific ridge which allows the
energy to slide inland, while the GFS tends to maintain a more
blocky ridge allowing for more amplification downstream across the
West. Yesterday 12z CMC was more like the ECMWF, although todays
00z run is more amplified like the 18z/00z GFS. The ensemble means
themselves seem to agree with their deterministic counterparts,
although there are a fair number of members showing both
possibilities. In other words, this aspect of the forecast remains
very uncertain.
WPCs forecast for tonight used a general blend of the
deterministic models through day 5, with a quicker than usual
transition towards the ensemble means days 6-7 (with smaller parts
of the both the ECMWF and GFS for a little more definition to
individual systems). This resulted is a good middle ground
compromise for the Western U.S. flow, which also maintained good
continuity with the previous WPC forecast through day 6.
...Weather/Threat Highlights...
The relentless heat across the Southern Plains/Lower Mississippi
Valley should finally break after Friday, though much warmer than
normal temperatures continue across the East into Saturday.
Numerous daily records could be met or exceeded still on Friday
across Texas/Louisiana and the Southeast on Friday, with a handful
of record values on Saturday in the East.
A strong cold front will spread for rain and thunderstorms to
spread through the Great Lakes/Ohio Valley/Mississippi Valley on
Friday and into the East on Saturday. Heavy rainfall as well as
severe weather is possible along this boundary given above normal
moisture and instability present. Flooding and flash flooding
could be an issue as well, particularly in places that may be
sensitive to additional rainfall given wet antecedent conditions.
Behind this front, unseasonably cool temperatures will slide into
the north-central U.S. Friday and Saturday, with highs generally
10-20F below normal. Parts of the northern-central High Plains
could be colder than that on Friday. This cold spell should lower
snow levels and possibly allow for some light snow to fall even in
northern parts of the High Plains. The Northwest will be fairly
dry late this week but may see some moisture return during the
weekend or early next week, with uncertain flow details keeping
confidence low for specifics of timing/coverage/amounts of any
precipitation. The southern half of the West should see a warming
trend expand from west to east Saturday-Monday with California
seeing the warmest high temperature anomalies of plus 10-15F.
Elsewhere, guidance agrees that tropical moisture should surge
northward across the Florida peninsula late this week into the
weekend, bringing a round of heavy rainfall to portions of the
Sunshine State.
Santorelli/Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are
at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml