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Quantitative Precipitation Forecast (QPF) Discussion
(Caution: Version displayed is not the latest version. - Issued 2105Z May 23, 2018)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
505 PM EDT Wed May 23 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 24/0000 UTC thru May 27/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1 Update...

Adjustments were made to the evening and overnight periods based
on the latest hires model consensus.  Overall, no significant
changes were made to the previous forecast.


Day 1

...Lower MS Valley---central Gulf coast into the
Southeast/southern Mid-Atlantic,,,

Widespread scattered diurnally enhanced convection likely day 1
along and to the south of the nearly stationary frontal boundary
expected to lie from the Lower MS Valley---across the central Gulf
coastal region and into the Southeast/southern Mid-Atlantic.  With
pw values expected to remain much above average along and south of
this front---locally heavy precip totals possible---although
confidence is low with details.  One region of potential max
precip will be in the vicinity of the nearly stationary surface to
mid level circulation in the vicinity of central Georgia.  Precip
may focus near this center and then to the west southwest and
northeast along the above mentioned frontal boundary position
and/or surface trofs separate from the frontal boundary positions
emanating from this nearly stationary low. 

...Northern Rockies--- Northern Plains into the Upper MS Valley...

Height falls pushing northward Wednesday across the Northern
Rockies will move on a more east northeast track Thursday across
the Northern Plains and toward the Upper MS Valley.  The model
consensus is for the well defined comma head/deformation precip
area currently stretching from the Northern Rockies into the
Northern High Plains to weaken at the beginning of the day 1 time
period with more scattered precip near the primary height fall
center over the Northern High Plains.  At the same time---broadly
difluent upper flow downstream of these height falls will support
scattered convection from the Northern Plains into the Upper MS
Valley.  Areal average moderate precip totals depicted with
locally heavier amounts likely where convection maximizes. 

...Central to Southern High Plains...

A lee trof/dry line will extend across a large area from the
Central to Southern High Plains day 1.  The broadly difluent upper
flow expected across the Northern Plains/Upper MS Valley will also
extend southward across the Central to Southern High Plains.  This
will support widespread scattered convection---primarily during
the first 6 to 9 hours of the day 1 period tonight and then again
late Thursday afternoon.  Confidence in qpf details is low in this
flow regime with a myriad of model solutions.  The HREF mean was
used to mitigate the model to model and model run to model run
differences.  Moderate areal average amounts depicted with locally
heavier totals likely where convection maximizes.

...Northern California---portions of the Pacific
Northwest---Northern Great Basin into the Northern Rockies...

Widespread diurnally driven shower and thunderstorm activity
likely early in the upcoming day 1 time period and again late in
the period during Thursday afternoon.  PW values expected to
remain above average across these regions in the wake of the lead
area of height fall ejecting north into the Northern Rockies early
day 1 and ahead of the developing closed low off the California
coast late day 1.  Generally light to moderate totals expected
across these areas--with isolated heavier totals possible.

Days 2/3...

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

Deep tropical moisture within confluent flow encounters a
weakening cold front over the southeast into Thursday night.
Diurnal heating produces an areal average QPF max of 1.5 inches in
eastern GA/southern SC east of a remnant 850 mb circulation. PW
anomaly of 2.5 immediately east of the 850mb circulation  warrants
a Slight risk for excessive rainfall for Day 2 (12Z Thu-12Z Fri)
with a surrounding Marginal risk.

Expect locally heavy showers in areas of diurnal instability and
sea/lake breeze initiated lift and convergence near the front,
particularly back over southeastern AL to the FL panhandle.

The models also depicted persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values near 2 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 for
Day 3 (12Z Fri-12Z Sat). 

QPF was based primarily on the 12Z GFS/ECMWF which are in good
agreement. The 12Z ECMWF continues a trend of slower arrival of
tropical moisture and circulation. The 12Z GFS continues a much
farther east track as if follows convective trends to FL. Much
uncertainty remains with this activity, but as noted, there is an
expanse of tropical moisture across the southeast, leading to at
least locally heavy rainfall.

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

A closed mid-upper level low off the CA coast Thursday night
drifts to NV through Saturday. Southerly flow ahead of the
circulations increases moisture with 700 mb convergence driving
rain and shower across northeast CA and northwest NV across into
OR. PW increases to around 1 inch ahead of the low over parts of
OR which is 2 standard deviations above normal, so expected
amounts are above normal for May. Thursday activity is expected to
propagate up the CA Cascades to OR. Slow storm motion warrants a
Day 2 (12Z Thu-12Z Fri) Marginal risk for excessive rainfall along
the CA/NV border into OR.

On Friday the deep layer circulation drifts onshore in CA with the
upper trough drifting east across OR.
Well defined coupled 700 mb convergence and 300 mb divergence
cross from northern Ca and NV into OR, with better than average
cluster of higher QPF across southern OR as a result. With the
east drift of the upper trough, the peak upper divergence drifts
into the OR Blue Mountains, so a secondary maxima is expected
there Fri/Fri evening. A Marginal risk for excessive rainfall is
in effect for much of southern OR. Good agreement among global
guidance nearly warranted a Slight risk, though an overall lack of
instability may limit rainfall. QPF was based on The 12Z ECMWF/GFS.

...Central Plains to the mid-upper MS Valley and upper Great

Activity will focus ahead of a low level front and prefrontal
moisture plume extending from northern KS across eastern Nebraska,
southeast SD, northwest IA and MN Thursday into Thursday night.
Clusters of showers and storms are expected again with convergence
on the nose of the low level jet progressing these areas. Given
the anomalous moisture plume pushing across the warm front (1.5
inch PW is 2.5 standard deviations above normal), a Marginal risk
for excessive rainfall was raised for Day 2 (12Z Thu-12Z Fri) over
central MN. This area will need to be monitored. It should be
noted that hi-res CAMs did not handle convective activity well
Wednesday morning across this area.

On Friday, the 700 mb trough move east from the northern and
central Plains towards the mid-upper MS Valley. Lift from the
approaching wave within the low level moisture plume and modest
low level jet maxima supports more showers/storms where
instability is focused. The east drift of the low level jet and
moisture plume and upper trough difluent flow brings the likely
targeted areas for shower/thunderstorms development in a corridor
from southeast KS to the UP of Michigan. A cold front over Ontario
is the terminus for tropical moisture, so much of the heavy rain
in Day 3 will occur in Canada. QPF was derived by blending the 12Z


Graphics available at