Quantitative Precipitation Forecast
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FXUS04 KWBC 242226

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
626 PM EDT Thu May 24 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 25/0000 UTC thru May 28/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Northern California---central to southern Oregon---northwest

Overall very good agreement on the slow east northeast movement of
the developing closed low off the central California coast--with
this system pushing inland Friday into central to northern
California.  Strongly difluent upper flow to the north and
northeast of this closed low will support increasing convection
Friday from northern California---across northwest NV and into
southern to central Oregon.  Much above average pw values in this
strongly upper difluent pattern will support heavy to isolated
excessive rainfall amounts across these areas.  No changes made to
the marginal risk on the Excessive Rainfall Outlook.

...Upper Great Lakes---Upper MS Valley---portions of the Great

A fairly large region of scattered convection possible day 1 ahead
of surface cold front expected to move slowly eastward across the
Great Plains.  Continued low confidence in this scattered
convection regime---with a large spread with respect to the
details of potential max qpf areas.  There is some model consensus
for a potential max region from central to eastern KS---northeast
OK into southwest MO and northwest AR.  Another possible max
region is over portions of the Upper MS Valley into the Upper
Lakes in a region of max isentropic lift along and to the
northeast of a northeast moving warm front.  With pw values
remaining above average across these areas---localized areas of
heavy precip possible where convection does maximize.

...Central Gulf coast into the Southeast...

Not a lot of changes expected to the recent pattern across the
Central Gulf coastal region and into the Southeast day 1. PW
values expected to remain 2+ standard deviations above the mean
along and to the south of the weakening stationary front draped
from the Lower MS Valley into the Southeast.  A nearly stationary
mid to upper level vort over central Georgia will continue to act
as a focus for heavy precip totals in the vicinity of this feature
over large portions of Georgia.  Somewhat higher confidence with
heavy precip totals near this feature---with lower confidence
elsewhere across the Central Gulf coast and Southeast.  There will
continue to be the potential for slow movement of cells and heavy
to locally excessive rainfall amounts across these areas day 1.

...Keys into South Florida...

Precipitation should be increasing day 1 across the Keys and South
Florida as the developing area of low pressure in the vicinity of
the Yucatan Peninsula begins to push northward.  With tropical
moisture being advected northward on the eastern side of this
low---the heavy precip threat will increase day 1---continuing
into the day 2 and 3 time period.  See the latest NHC outlooks or
potential future advisories for additional information on this

...Northern Maine...

Height falls dropping southeast across central QB will support the
potential for several waves of precip to streak in a west
northwest to east southeasterly direction across northern Maine
this period.  The potential for several waves of precip will
support moderate to locally heavy totals---with the latest models
showing fairly good agreement here.

Days 2/3...

...Southeast/eastern to central Gulf Coast/FL...

Deep tropical moisture within confluent flow continues to combine
with diurnal heating/destabilization to produce widespread
showers/storms from southern SC to central AL along a weakening
surface front under a remnant 850 mb circulation over GA. Above
normal moisture leads to a continuation of a Slight Risk for
excessive rain over GA for Day 2 (12Z Fri-12Z Sat).

Guidance continue to depict persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values of 2 to 2.25 inches so showery
conditions are expected along the southeast coast. The pattern
favors inland penetration of the sea breeze from the east coast so
peak afternoon convection should occur over interior portions of
the FL peninsula to the west coast. Lapse rates are not as high as
usual due to warm mid level temps, so bands of larger scale lift
are needed to trigger FL convection. Convective activity into
southern FL warrants a Marginal risk for excessive rainfall there
for both Days 2 and 3 given recent wet conditions.

Trends on the advection of the tropical moisture and embedded
convective activity continue with the GFS trending west and the
ECMWF still trending slower. A blend of the 12Z GFS/ECMWF was used
for QPF.


Upper level ridging over the eastern CONUS will allow tropical
moisture to continue to lift north from the southeast, reaching
the Mid-Atlantic Friday night into Saturday. A cold front will
shift south across the northeast. This simple low level
convergence of much above normal moisture (reaches 2.5 standard
deviation over the central Mid-Atlantic on Sunday) may cause a
predecessor rainfall event over a portion of the Mid-Atlantic late
this weekend into next week. The 12Z ECMWF greatly increased the
QPF east of NJ Saturday night into Sunday. Signals like this will
need to continue to be monitored for what is a difficult to
pinpoint phenomena (predecessor rainfall events).

...Pacific Northwest/Great Basin/Northern Rockies...

A closed mid-upper level low drifts onshore and from CA to UT
Friday night through Sunday. Southerly flow ahead of the
circulation maintains above normal moisture. Continued the
Marginal risk for excessive rainfall over OR/far northern CA for
Day 2 (12Z Fri-12Z Sat) where 0.75 to 1 inch PW persists with low
to mid-level convergence.

Saturday the 700 mb circulation moves northeast out of NV towards
the WY/UT border. Continuing 700 mb convergence along and
northwest of the low track promotes showers across northeast NV
across southern ID and much of MT. With low pressure developing on
the northern Rockies, low level convergence leads to showers
developing over the Rockies into the northern high Plains Saturday
night. QPF was based on the 12Z GFS/ECMWF.

...Great Lakes...

Activity Friday night into Saturday will focus ahead of a low
level front and prefrontal moisture plume with low-mid level
convergence on the nose of the low level jet over mid-upper MS
Valley/northern Great Lakes. Locally excessive rainfall is likely
again with the plume of sub-tropical moisture extending across
this area into Canada. No excessive risk was raised at this time,
though that may need to change for Day 1. QPF was based on the 12Z


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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