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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1833Z Jun 13, 2019)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
233 PM EDT Thu Jun 13 2019

Valid 12Z Sun Jun 16 2019 - 12Z Thu Jun 20 2019

16Z Update: 
The WPC forecast for the medium range period remains very similar
to the previous forecast issuance.  The main forecast challenges
are the timing of individual shortwave perturbations and how they
affect potential waves of low pressure developing along a
quasi-stationary frontal boundary across the Plains and Ohio
Valley.  Another challenge is the position and orientation of a
broad trough axis over the central-western U.S. by later in the
forecast period, with GEFS ensemble members suggesting the trough
becoming established over the southwestern U.S., and the EC
ensemble members farther east across the Plains.  After running
several teleconnections with multiple height anomaly centers over
the eastern Pacific and northern Canada, a solution closer in line
with the GEFS mean regarding the upper trough seems most likely,
and the forecast for Wednesday and Thursday of next week was
weighted slightly more towards the GEFS compared to the EC mean. 
Earlier in the forecast period, there was enough synoptic scale
agreement to merit a blend of the GFS/ECMWF/CMC/UKMET, with a
greater emphasis on the GFS and ECMWF. /Hamrick

...Overview and Guidance/Predictability Assessment...

Latest guidance maintains the idea of broadly cyclonic mean flow
aloft over the lower 48, between ridges over the western Atlantic
and eastern Pacific.  Consensus still shows robust strengthening
of the Pacific ridge from late weekend into early next week.  This
ridge may ultimately promote more energetic troughing over parts
of the western-central U.S. by next Tue-Thu in contrast to the
weaker/more diffuse features likely to be embedded within the mean
flow early in the period. 

The overall pattern should lead to broad coverage of rainfall from
the Rockies eastward.  Details at any particular point in time are
still quite uncertain but the general signal for heaviest rainfall
potential within an area from the southern half of the Plains into
the Ohio Valley/central Appalachians has been consistent over
multiple days.  The setup appears favorable as a decelerating wavy
front settles into a Northeast U.S. to southern Plains orientation
during the Sun-Tue period while steady low level inflow from the
Gulf provides ample moisture, and one or more shortwaves aloft
likely encourage convective development.  The front should return
north as a warm front late Tue onward as a system emerges over the
Plains.  This latter system should bring some enhanced rainfall to
areas across the northern tier as well.  The additional rainfall
would be problematic over areas already experiencing flooding or
have wet ground.

Guidance continues to vary for the precise handling of shortwave
energy progressing eastward from the northern Plains Sun onward,
keeping confidence low for specifics of a general area of waviness
likely to track from the Midwest/Lower Great Lakes into the
Northeast during the first half of the period.  Separate shortwave
energy tracking out of the central Plains could have an influence
on the forecast as well.  Prefer maintaining a blend approach to
reflect the most common themes of guidance without committing to
any extreme aspects of some individual solutions.   

Farther west the greatest forecast problem involves the character
of the Pacific ridge and specifics of energy expected to amplify
into the downstream trough.  The past couple ECMWF runs through
12Z June 12 were the quickest to flatten the ridge (by way of
upstream trough progression), leading to faster system timing over
the northern states by Wed-Thu.  Although the new 00Z GFS (note
the FV3 GFS become the operational run as of the 12Z June 12 run)
adjusted toward a slower variation of the ECMWF, strength of the
ridge by mid-period seems to favor leaning away from any solution
that is quickest to weaken it.  Beyond the issue of the ridge,
there are also various possibilities for details of energy around
the top of the ridge and feeding into the downstream trough.  The
12Z ECMWF mean is most enthusiastic to bring the upper low just
south of Alaska bodily southeastward.  Remaining solutions show
greater potential for the upper low to hang farther back but at
the same time have energy flowing to its south amplify into the
trough.  The GEFS mean becomes slowest with the trough. 
Ultimately favored a blended approach (increasing ensemble input
while reducing/removing weight of the 12Z ECMWF with time) that
led to an intermediate solution closest in principle to a deeper
version of the 12Z CMC mean. 


...Sensible Weather Highlights/Threats...

The expected pattern evolution at the surface and aloft continues
to be favorable for multiple episodes of convection/heavy rainfall
from the southern half of the Plains into the Ohio
Valley/Appalachians.  However there is still considerable
uncertainty for some of the day-to-day details, and in fact
guidance spread over the southern Plains appears to have increased
compared to previous days.  As a result the potential for highest
five-day totals within the full threat area seems to be more
evenly distributed versus concentrating more toward the Plains in
previous days.  Some of the activity across this area will likely
extend into the Northeast/Mid-Atlantic with locally moderate-heavy
rainfall possible.  It will likely take fairly far into the short
range time frame to resolve the specifics given the low
predictability of important small scale features and the
difficulty that models sometimes have in representing convection. 
Over the West the best concentration of showers/thunderstorms
should be over the northern-central Rockies.  Coverage and
intensity become increasingly uncertain by Wed-Thu given spread
for details of flow aloft.  During Tue-Thu expect rainfall of
varying intensity to spread into the northern Plains/Upper
Mississippi Valley with the system forecast to affect those
regions.  The Florida peninsula should see diurnal convection
through the period.

Over the West expect broad coverage of above normal temperatures
(greater for morning lows than daytime highs) Sun-Mon with some
pockets of greater than plus 10F anomalies.  Readings should then
trend cooler as jet energy reaches the Northwest by Tue, leaving
parts of California as the primary area of warmth heading into
midweek.  Areas across the northern/central Rockies and Plains
will tend to see moderately below normal highs, perhaps trending
cooler over northern locations late in the period.  Over the East
areas of clouds/rainfall should favor near to above normal lows
but near to slightly below average highs.

Rausch


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain across portions of the Central/Southern Plains, the
Middle/Lower Mississippi Valley, the Ohio Valley, the Tennessee
Valley, the Central/Southern Appalachians, the Great Lakes, the
Mid-Atlantic, and the Northeast, Sun-Thu, Jun 16-Jun 20.
- Flooding possible across portions of the Northern Plains.
- Flooding occurring or imminent across portions of the
Northern/Central Plains, the Middle/Lower Mississippi Valley, the
Mid-Atlantic, the Southeast, the Great Lakes, and the Ohio Valley.
- Flooding likely across portions of the Central Rockies and the
Central Great Basin.
- Excessive heat across portions of the Southern Plains, Sun-Thu,
Jun 16-Jun 20.
- Much above normal temperatures across portions of the Northern
Rockies and the Northern Great Basin, Sun-Mon, Jun 16-Jun 17.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml