Quantitative Precipitation Forecast
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FXUS04 KWBC 222111

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
510 PM EDT Tue May 22 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 23/0000 UTC thru May 26/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1 Update...

For the evening and overnight periods, adjustments to the prelim
Day 1 forecast were based largely on recent runs of the
operational and experimental HRRR, along with the HREF model
consensus.  For the Wed morning and afternoon periods, they were
based largely on the HREF model consensus.  One of more
significant changes was across eastern South Dakota into southern
Minnesota and northern Iowa during the overnight periods.  While
remaining south of some of the hi-res models, including the NAM
CONEST and NSSL-WRF, did adjust the axis of heavier amounts
further to the northeast based on recent runs of the operational
and experimental HRRR, as well as the HREF Mean.  Also extended
heavier amounts more quickly downstream across northern Iowa
during this period as well.  While not quite to the degree of the
recent HRRR runs, also increased areal average amounts back across
western South Dakota during the overnight period.


Day 1...

...SD/MN/IA border junction...
Tonight should be a good setup for an axis of elevated convection
with heavy rains across portions eastern SD into southwest MN and
northwest IA where a broad region of low-level inflow intersects
and overrides a warm front.  Elevation over the surface boundary
might not be by much as MUCAPE values are expected to start in the
1500-2000 J/kg range at 06z and the best 850 hPa wind
convergence/confluence appears to be near the SD/IA border.  The
850 mb moisture transport increases and persists through the night
downwind of the instability pool.  Storm motion vectors are at a
right angle to the low-level inflow expected, suggesting that
storms could regenerate and train for a period from west-northwest
to east-southeast.  Still a good amount of uncertainty with model
placement, though there is a strong enough signal for 3-5" totals.
 Think a solution somewhat south of the 12z ARW is in the ballpark
of what could materialize.

...Northern Rockies/Divide...
Upslope flow northeast of a deep layer cyclone combined with
precipitable water values of 0.75-1" are expected to focus heavy
precipitation near the western WY/MT border into eastern ID.  For
the most part, continuity resembled recent guidance, so let it
ride.  Increased amounts somewhat in the Wind River Range per the
most recent guidance.

A wet pattern is expected across the Southeast and Mid-Atlantic
States.  The main foci are the low-level circulation which moved
inland from the Gulf yesterday into eastern AL and a frontal
boundary shifting southward through the region.  This afternoon,
heavy rains are expected to start in north-central VA and southern
MD and shift east and southeast this afternoon and evening before
fading overnight near the VA/NC border.  The guidance has trended
in this direction so have followed suit.  The portion of the
pattern which most resembled continuity was with the eastern AL
circulation which is expected to spread heavy rains into GA.  The
flow pattern in FL suggests a west coast sea breeze maximum, which
resembled continuity which was strongly maintained.

...Southwest OR/northern CA...
Diurnal convection each afternoon and evening is expected near and
downwind of the terrain within the western periphery of a deep
layer low.  Increased amounts somewhat across this area to edge
continuity towards the latest model guidance.

An upper low near the southern LA/TX border should act to focus
some convection over portions of AR/LA/MS through tomorrow within
a region of relatively weak flow.  Diurnal convection with brief
heavy rainfall is possible here before the storms pulse/become
outflow dominant.  While very localized heavy totals are possible,
areal averaged rainfall should not be all that high.

...NM/TX border...
Followed the guidance trend to shift activity farther southward
within a regime with shortwaves rotating east to northeast around
the southern periphery of a deep layer cyclone across the Great

Days 2/3...

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

Deep tropical moisture within confluent flow encounters a cold
front moving across the southeast from the north, particularly
Thursday. Diurnal heating produces an areal average QPF max of
only around one inch on Wednesday, but given recent and ongoing
activity in the area, as well as slow storm motion per 850 to 300
mb mean wind of about 5 kt, heavy rain can be expected. A marginal
risk for excessive rainfall was raised for Day 2 (12Z Wed-12Z Thu)
along the southern AL/GA border. Elsewhere expect locally heavy
showers in areas of diurnal instability and sea/lake breeze
initiated lift and convergence near the front.

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.

QPF was based primarily on the 12Z GFS and some 12Z ECMWF which
came into better alignment over the central Gulf coast compared to
the 00Z ECMWF.  The 12Z NAM continues to be an outlier bringing
low pressure closer to the central Gulf coast much sooner that
other solutions so the NAM was given no weight.

The cold front weakens for Day 3 (12Z Thu-12Z Fri) and the average
QPF max shifts to south-central GA where less rainfall has
occurred in the past week than along the AL/GA border, so no
excessive rainfall risk was raised.

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

An amplified trough off the entire west coast Wednesday evening
closes into an upper low and reaches the northern CA coast Friday.
PW increased to around 1 inch ahead of the low over OR/WA which is
2 standard deviations above normal, so an excessive rainfall risk
is possible by Day 4.

In advance, an upper level trough and corresponding 700 mb low
over MT Wednesday evening will lift northeast into the Canadian
Prairies Thursday.

Instability under the trough will allow convective showers and 1
inch PW is 2 to 2.5 standard deviations above normal, so amounts
are higher than you typically see with a late May system. Showers
occur in the area of low-mid level convergence and ascent near the
low, which combined with terrain near Yellowstone and snow melt
warrants a marginal risk for excessive rainfall for Day 2 (12Z
Wed-12Z Thu). The focus for heavy rain shifts to the Canadian
Prairies with the low on Thursday. QPF was mainly based on the 12Z

...Eastern Northern Plains/Upper Midwest...

A warm front will focus convection on a plume of moisture with the
ECMWF forecasting near 1.5 inches precipitable water at its peak
as it lifts north across MN Wednesday night and Thursday. Activity
will focus ahead of a lee trough in the high Plains, with another
plume of enhanced moisture forecast to form ahead of the trough
and approaching cold front Thu evening in MN. Clusters of showers
and storms are expected again with convergence on the nose of the
low level jet progressing across MN, areas near the SD/IA border,
and eastern Nebraska. PW is 2 to 3 standard deviations above
normal, but the main convergence is expected to lift into Canada
with a limited threat for excessive rainfall over the northern
plains and upper Midwest. QPF based on the 12Z ECMWF/GFS.

...West Texas/eastern NM/TX-OK panhandles into west KS...

Another deepening low approaches CA Thursday.  The trough over NM
lifts out is replaced by a building mid-upper level ridge.
Isolated to scattered showers/storms are possible but with less
coverage due to weakened low level convergence and lift. The 12Z
GFS continues to demonstrate convective feedback, but locally
heavy showers and thunderstorms can again be expected into
Wednesday before the forcing lifts well north.


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


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