Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 PM EDT Mon Jun 17 2024

Valid 12Z Thu Jun 20 2024 - 12Z Mon Jun 24 2024

...Record-breaking heat from the Midwest to the East Coast...

...Heavy rainfall threat over the northern Plains/Upper Midwest
late week and southern High Plains on Thursday...


...Overview...

During the Thursday-Monday period, guidance is showing a steady
transition from a strong East Coast upper ridge toward slightly
weaker and more elongated upper ridging that settles across the
southern tier while flatter mean flow becomes established across
the northern states. The initial upper ridge will support a broad
area of hazardous heat with potential for daily (and possibly
monthly) records extending from the Ohio Valley/Lower Great Lakes
into the northern Mid-Atlantic and Northeast, with the pattern
evolution helping to moderate temperatures with time especially
over northern areas. Meanwhile the northern Plains/Upper Midwest
may see a continued threat for heavy rainfall through late this
week as ejecting West Coast trough energy develops northern Plains
low pressure that will aid the interaction of anomalous moisture
and a leading front. Some of this rainfall will extend into the
Northeast. A trailing Northeast Pacific system will likely bring a
well-defined front into the West by Sunday. To the south of the
upper ridge, upper level energy and abundant deep moisture north of
an area of low pressure (see NHC for updates) will bring the
potential for a period of heavy rain over the southern High Plains
and vicinity by Thursday.


...Guidance/Predictability Assessment...

Latest models and ensembles show overall good agreement on the
upper level pattern through the medium range period, but there
remain some key differences in the details. There is some
uncertainty still in the westward extend of the Eastern ridge early
in the period, but a general model blend seemed to provide a good
starting point. By the weekend, a shortwave through the northern
Plains/Upper Midwest should act to suppress the ridge southward.
The 00z CMC was a little different in its evolution of this
shortwave into the Great Lakes/Northeast around Sunday due to
resolving of details as multiple pieces of energy over central
Canada combine in the the overall shortwave trough. The CMC was
also notably slower with the next trough into the Northwest early
next week, but the 12z run (available after forecast generation
time) did trend faster and more in line with the GFS and ECMWF.


Todays updated forecast was based on a general blend of the latest
deterministic guidance (06z GFS and the 00z ECMWF, CMC, and UKMET)
for the first half of the period. By Sunday-Monday, continued to
use 60 percent of the deterministic GFS/ECMWF with 40 percent of
the ensemble means to help mitigate some of the differences late in
the period. Overall, this maintained good agreement with the
previous WPC forecast as well.


...Weather/Hazards Highlights...

Excessive Rainfall Outlooks for both Day 4 and Day 5 (covering
Thursday through Friday night) continue to depict a Slight Risk
area covering parts of the northern Plains/Upper Midwest. There has
been a persistent signal in the guidance for convection to persist
over this region from the short term, with the Days 4-5 period
featuring gradual northward progression of a warm front as ejecting
West Coast trough energy develops a defined surface wave. In
addition to the favorable moisture/synoptic ingredients during the
outlook period, prior days of rainfall will lead to wet ground
conditions heading into Thursday.

Farther south, the Day 4 outlook made minimal change to the Slight
Risk area depicted near/east of the Big Bend region of Texas, as
guidance continues to show an axis of significant moisture
anomalies moving eastward across the region. The outlook reflects a
relative majority/intermediate solution among the ongoing spread
for rainfall magnitude and south-north coverage.

Elsewhere, the Days 4-5 ERO outlooks continue Marginal Risk areas
over parts of the southern Rockies/Four Corners region due to the
abundant moisture arriving from the east and some forecast
instability, along with some model signals for locally enhanced
rainfall. Guidance signals are trending more diffuse for rainfall
potential along the Southeast coast with a possible wave/moisture
arriving around Thursday, so the Day 4 outlook has removed the
previous Marginal Risk area. By Friday, the Northeast may merit
some monitoring as a front settles over the region.
Moisture/instability could support some locally enhanced rainfall
rates but antecedent conditions should be neutral to dry by that
time and model QPF is not exceptionally high thus far, so the
outlook depicts no risk area at this time.

From Saturday onward the area of heavy northern tier rainfall
should progress eastward into the Great Lakes/New England along
with the supporting upper trough and surface system. A broader area
of somewhat lighter rainfall will be possible along and ahead of
the trailing front between the Plains and East Coast. Moisture
lingering over parts of the Rockies may support continued episodes
of diurnal convection. The front reaching the Northwest by Sunday-
Monday may produce a little light/scattered rainfall.

The strong upper ridge initially over the East should produce the
greatest temperature anomalies from the Midwest and Ohio
Valley/Lower Great Lakes into the northern Mid-Atlantic and
Northeast Thursday-Friday, with multiple days of readings 10-20F
above normal. This would translate to highs well into the 90s,
along with lows in the upper 60s to upper 70s providing little
overnight heat relief. Daily records will be possible within the
above areas, with some isolated monthly records possible as well.
Gradual flattening of the upper pattern should begin to lower
temperatures over far northern areas by Friday-Saturday but
otherwise a decent area of plus 10-15F anomalies should persist
through the weekend. Meanwhile much of the West will trend
warmer/hotter late this week into the weekend with highs reaching
10F or more above normal for multiple days. The front reaching the
Northwest should bring cooler air to that region by Sunday-Monday,
while some of the western heat should reach the northern-central
High Plains at that time. Clouds/rainfall will support below normal
highs over the southern Rockies/High Plains late this week.

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 forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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
https://www.wpc.ncep.noaa.gov/#page=ovw