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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0656Z May 19, 2018)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Sat May 19 2018

Valid 12Z Tue May 22 2018 - 12Z Sat May 26 2018

...Guidance and Predictability Assessment...

Latest model/ensemble solutions show reasonable similarity for the
overall mean flow but still present some meaningful uncertainties
for specifics over some areas. 

One of the major question marks has been how eastern Pacific flow
splits by the middle of next week and what the downstream impact
on flow details will be.  At the moment latest runs signal a
compromise of sorts relative to the wide spread from 24 hours ago.
 This compromise idea involves a closed low evolving over the
Pacific farther offshore than in some ECMWF runs before the 12z
cycle but not as far west as some earlier GFS runs, while northern
energy briefly dents the western Canada mean ridge to a greater
extent than in the prior ECMWF/ensemble mean cluster but without
the more significant breakdown that had been advertised in some
earlier GFS runs.  Among GFS runs through 18z, the 18z version
compared much more closely to the majority cluster than the 12z
version.  New 00z runs maintain decent continuity in principle but
still display significant spread in how quickly height falls
approach the West Coast, with recent trends toward a compromise
(and ensemble means showing a partial trend toward operational
runs) seemingly recommending an intermediate solution.  Forecast
confidence has improved from yesterday but there is still enough
sensitivity in the evolution to suggest significant changes in
guidance are still possible.

Ahead of this evolution there is decent continuity/consensus with
the initial western U.S. upper trough and embedded upper low
weakening while it ejects northward and then eastward, likely to
head into the northern Plains by late in the week.  In the wake of
this feature heights aloft will rise over the West with a strong
ridge also building over Mexico.  With understandable detail
differences there is a decent signal in the guidance that some
combination of the late week northern Plains shortwave and energy
progressing through western Canada will promote a general area of
low pressure over southern Canada and the vicinity of the upper MS
Valley by early day 7 Sat.

Over the eastern states, guidance is still having difficulty
latching onto specifics of weak waviness over the OH Valley/lower
Great Lakes/Northeast Tue-Wed.  Recent trends have been toward a
weaker reflection with a general model blend of 12z/18z guidance
offering the best starting point given its fairly small
scale/lower predictability.  An upper trough passing through
eastern Canada and brushing the Northeast will push a cold front
into parts of the East immediately behind the initial wave. 
Southward extent of this front will depend on timing/amplitude of
the upper trough, with models/ensembles not yet decided on
specifics and no coherent multi-run trends seen thus far.

Latest guidance still offers very diverse possibilities for low
pressure that may track out of the northwestern Caribbean.  GEFS
members have tended to be on the fast/eastern side of the spread
while CMC runs have also been fairly progressive but on the
western side of the envelope.  Combined uncertainties including
degree/depth of development determining what level of steering
flow will have greatest influence at a particular time, along with
evolution/position of forecast upper troughing over the southern
U.S./Gulf of Mexico, lead to continued very low confidence for
details of this feature.

Based on the composite of forecast considerations through the 18z
cycle of data, the early part of the forecast incorporated mostly
operational model input with greater weight on the 18z GFS/12z
ECMWF.  Increasing uncertainty with time favored increased
ensemble mean weight (slightly more than half of the overall
blend) by mid-late period but with more ECMWF mean versus GEFS due
to the latter not being favored over Florida and vicinity.

...Weather/Threats Highlights...

The upper trough/low ejecting out of the West and into the
northern Plains will likely promote areas of precip through the
period, from portions of the northern half to two thirds of the
West and into the central U.S.  This primary feature along with
leading impulses will likely interact with a surface front
anchored by central High Plains low pressure to produce multiple
convective episodes.  Best signal for highest rainfall totals
during the period extend from the northern/north-central Rockies
toward the upper half of the MS Valley.  Near the West Coast late
in the period, the upper trough/low offshore may get close enough
to generate mostly light/scattered precip but confidence is low. 
Meanwhile the Tue-Wed system over the East will likely be
accompanied by areas of rainfall of varying intensity.  South of
the front that likely stalls over the mid latitudes of the lower
48, there will be periods of diurnally favored showers/storms. 
Forecast specifics for Florida and vicinity remain very uncertain
given low predictability of the feature that may emerge from the
northwestern Caribbean.  There is still somewhat better confidence
that the overall pattern should promote an increase of moisture
and potential for some areas of locally moderate to heavy rainfall.

Much of the country will see above normal temperatures during the
period.  Greatest anomalies should be for min/max readings from
the Pacific Northwest eastward along the Canadian border and for
min temps over parts of the East during the first half of the
period.  Another band of relatively high anomalies may set up over
the central Plains/MS Valley.  Upper troughing will promote below
normal highs over the Great Basin/Southwest early in the period
while clouds/rainfall will tend to keep Florida highs slightly
below normal.

WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at: