457
FXUS02 KWBC 170659
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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...

The updated forecast based on 12Z/18Z guidance tilted more toward
the operational models relative to minority ensemble mean input
early, followed by a gradual increase of the means toward 60
percent by the end of the period next Monday. Low confidence detail
differences led to splitting the GFS among 12Z/18Z runs by late in
the period.

ECMWF-initialized machine learning (ML) models have continued to
suggest that the eastern U.S. upper ridge may ultimately remain
centered farther west than what some dynamical guidance has been
suggesting in recent runs. In the 12Z cycle, the CMC/CMCens were
closest to the ML theme so those solutions were given a little more
weight in the forecast blend than would ordinarily be the case,
though not quite to the point of being in the majority. The new 00Z
GFS/GEFS seem to be trending that way as well. Meanwhile latest
ECMWF runs are running weaker than most other dynamical and ML
guidance for the upper ridge by mid-late period, with more
influence from embedded/surrounding upper energy.

There is decent agreement in principle but continued detail
differences for the northern tier upper trough/surface system
associated with initially ejecting West Coast trough energy as well
as the Northeast Pacific system that eventually reaches into
western North America. The latter actually has reasonable
clustering into early Sunday, with both dynamical/ML models
diverging thereafter. Most guidance suggests that latest GFS runs
may become too strung out/progressive with the incoming energy by
next Monday.

...Weather/Hazards Highlights...

Excessive Rainfall Outlooks for both Day 4 and Day 5 (covering
Thursday through Friday night) have introduced 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. Latest
guidance appears to show enough coherence in the location of best
convective focus to merit introduction of the Slight Risk area. 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 coverage. The ECMWF/UKMET remain the
most suppressed to the south while other dynamical guidance plus
ECMWF-initialized ML models show somewhat more northward extent.

Elsewhere, the Days 4-5 outlooks have introduced 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 reduced the
coverage of the existing Marginal Risk area as a lead-in to
eventual elimination if guidance continues the drier trend. 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.

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



































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