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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0655Z Jun 21, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Jun 21 2024

Valid 12Z Mon Jun 24 2024 - 12Z Fri Jun 28 2024

...Hazardous heat possible much of next week for the
South/Plains...


...Overview...

An upper level ridge will remain anchored over much of the
Southern half of the nation through the week, leading to multiple
days of above normal to possibly hazardous heat. To the north,
individual shortwaves within the westerlies crossing southern
Canada and the northern tier of the U.S. to the Northeast will
support a series of progressive surface lows and trailing fronts
across the northern half to two thirds of the lower 48. A deeper
upper low will build off the Northwest coast, moving into the
region sometime later in the week.


...Guidance/Predictability Assessment...

The latest dynamical and machine learning (ML) guidance agrees
fairly well for the overall pattern but displays typical spread and
run- to-run variability for the details. Latest runs of the ECMWF
remain on the slightly slower side for an initial shortwave through
the Northeast Monday-Tuesday, while the GFS is slightly stronger
with the next one near the Great Lakes at the same time. The
greatest region of uncertainty was out west with a building trough
off the Northwest Coast. The 18z/Jun 20 GFS/GEFS guidance was quite
a bit faster than the ECMWF/CMC to bring it inland. The ensembles
and ML models generally supported something in the middle. Trends
in the new 00z guidance for tonight (available after forecast
generation time) are favorable, with the ECMWF/CMC a bit faster,
and the GFS slower.

WPC forecasts for tonight favored a majority deterministic model
blend for the first half of the period amidst relatively good
agreement. Trended away from the fast 18z GFS and more towards the
ensemble means late period for the Western U.S. system and
elsewhere. Overall, this maintained good continuity with the
previous WPC forecast as well.


...Weather/Hazards Highlights...

The greatest focus for possible impactful rainfall/convection will
be along a frontal boundary as it shifts eastward from the Plains
to the East Coast Monday-Wednesday. There is a marginal risk
highlighted on the Day 4 Excessive Rainfall Outlook (Monday- Monday
night) from far eastern South Dakota to western Michigan where
copious moisture and instability should be present in an area that
has seen heavy rainfall in past days and may see more in the short
range period. The threat for excessive rain will shift south and
east on Tuesday with a broad marginal risk depicted from the south-
central Plains, across the Middle Mississippi Valley, into the
Upper Great Lakes on the Day 5 ERO where a typical summertime
convective pattern should emerge along and ahead of the front.
Convection and at least locally heavy rainfall should accompany the
front as it pushes into the East by Wednesday as well.


The southeastern Canada surface low and trailing front affecting
the Northeast on Sunday and perhaps to a lesser degree into Monday
will have the potential to produce some areas of locally heavy
rainfall across the region. There is continued spread in the
guidance for exactly how this system will evolve and thus for
rainfall details. The Day 4 Excessive Rainfall Outlook covering
Sunday-Sunday night maintains a Slight Risk area over parts of
northern-central New England with modest adjustments based on
latest model/ensemble guidance and first-guess fields. The
surrounding Marginal Risk that extends as far south as the central
Appalachians is also close to continuity and reflects the
combination of potential for locally heavy activity and terrain
sensitivity. The Day 4 ERO maintains a Marginal Risk area over far
southern Texas where moist flow to the north of a second possible
western Gulf surface low may produce some enhanced rainfall. It is
more likely than not that impactful rain will stay south out of
Texas, but a low-end chance makes maintaining a Marginal for far
South Texas reasonable. In the day 5 ERO period (Monday-Monday
night), guidance continues to suggest an area of possible
convection diving southeast over the Upper Midwest/western Great
Lakes ahead of an advancing warm front. A Marginal Risk remains a
good starting point, as downsides for confidence in flash flooding
are relatively fast movement and model spread with placement, but
upsides are high rainfall rates and wet ground conditions.

Anomalous moisture and sufficient instability should persist over
the Four Corners states during the Days 4-5 period and potentially
lead to some locally heavy rainfall, particularly over burn scars.
Marginal Risks are in the outlook across parts of Arizona and New
Mexico where the best moisture transport is for slow- moving
convection. Parts of the Southeast/Florida may also see episodes of
diurnally enhanced convection, but overall dry conditions/high
FFGs precluded any excessive rainfall risk areas at this time. A
late- period front reaching the Northwest may produce some light
rainfall Wednesday into Thursday.

For the first half of the week, above normal temperatures will
stretch from the West to the East, though the focus will be across
the central Plains states into the Southeast where daytime highs
10-15F above normal will be present, and the combo heat/humidity
could lead to heat indices near 110F for some, and widespread major
to extreme heat risk. Hazardous heat is also possible across the
Southwest where heat indices 105-110F are likely. Temperatures
should moderate back towards normal for the East and West states as
upper troughing moves through both regions.


Santorelli


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