Quantitative Precipitation Forecast
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FXUS04 KWBC 142100

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
500 PM EDT Mon May 14 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 15/0000 UTC thru May 18/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Upper Mississippi Valley---Lower Great Lakes---Mid
Atlantic---NY state into New England...

Active convection expected along and ahead of the mean frontal
boundary position expected to lie in a general west to east
direction from portions of the Upper MS Valley---through the Lower
Great Lakes---Mid-Atlantic into New England.  There is expected to
be very favorable right entrance region jet dynamics across these
regions in an axis of pw values 1.5 to 2.5 standard deviations
above the mean.   Convection likely to enhance in the vicinity of
this front this evening over the Upper MS Valley as moist
southwest flow strengthens into the above mentioned west to east
boundary--with this activity pushing downstream into the lower
Great Lakes region overnight.  While this activity may weaken by
or prior to 1200 utc Tuesday...additional convective enhancement
likely Tuesday afternoon from the eastern Great Lakes---thru NY
state---New England and the Mid-Atlantic.  Widespread moderate to
heavy rainfall potential possible across these areas.  The href
mean and in house hi res mean used for the details day 1 to
mitigate the large spread evident in the latest models.

...Florida into the Southeast...

The mid to upper level closed low over the eastern Gulf of Mexico
expected to move only slowly northward day 1 along the northwest
Florida coast into the western panhandle region.  There continues
to be a strong model signal for widespread moderate to heavy
precipitation potential across large portions of the Florida
peninsula into the Southeast.  Model consensus for max values are
near and just north of the mid to upper level closed low over the
central to eastern portions of the Florida panhandle where 2-4"+
totals are possible.  A second max may align from northern Florida
into the Southeast where a low level convergence axis maximizes to
east northeast of the associated surface low.  Outside these two
max areas---confidence lowers with qpf details across the
remainder of the peninsula.  However---with pw values still above
average---albeit not as high as in the two above max
regions---locally heavy rains are possible here also.

...Southern Plains into the Lower MO Valley...

Low confidence this period with qpf details from what should be
active scattered convection along and ahead of the dry line over
the Southern Plains and then farther northeastward along the
western extent of the frontal boundary stretching from the Upper
MS Valley into the Northeast.  Very difluent upper flow likely
downstream of the mean Great Basin closed low across these areas.
This will be occurring in an axis of pw values in the 1.5-1.75"+
range---supporting locally heavy rainfall potential.  Run to run
and model to model spread is large---leading to the above
mentioned low confidence.  The href mean and in house hi res means
were also used here primarily for qpf details given this spread.

...Sierra---northern California---Great Basin...

The Great Basin closed low over central NV is expected to remain
stationary for the first half of day 1---then lift northward
Tuesday in response to upstream height falls approaching the
central California coast.  Models are consistent in showing a
broad region of scattered convection in the vicinity of this
closed low from northern California---thru the Sierra and into
Great Basin.  Areal average light to moderate totals
possible---with the best chance of any isolated heavy totals being
through the Sierra into the northwest California coastal range.

..Lee of the Central Rockies...

Scattered convection possible late Monday afternoon into this
evening in a region of post frontal east northeasterly upslope
flow in the lee of the Central Rockies.  The spread in qpf details
is typically fairly large in this flow regime.  Areal average
moderate totals depicted with isolated heavy amounts possible
where convection maximizes early Tuesday morning.

Days 2/3...

...Pacific Northwest and Northern California to the Northern

Shower and thunderstorm activity will increase over portions of
the Pacific Northwest, Northern California, and the northern
Rockies Tuesday night into Wednesday and Thursday as a weakening
upper low lifts out the central Great Basin and a second upper low
edges inland over California.  Sufficient moisture and instability
across the region will support moderate to locally heavy
precipitation, and model guidance is in general agreement that the
highest totals should focus in an axis extending from Northeastern
California and southwestern Oregon into northern Idaho and
northwestern Montana.  No major changes were made to the WPC QPF
or Day 3 marginal risk across northeast Oregon, Idaho, and
northwestern Montana.

...Northern Plains and Upper Mississippi Valley...

Moisture and instability expected to funnel along a frontal
boundary extending from eastern Montana into North Dakota and
Minnesota should fuel convection developing across the region
early Thursday.  An axis extending from central North Dakota into
central to northern Minnesota is the current model consensus for
the highest precipitation totals.

...Southern to Central Plains...

A ripple of shortwave energy ejecting out of the southern Rockies
should provide synoptic scale ascent for convection developing
ahead of a weakening front/dry line extending across the western
Red River Valley during Day 2.

...Mid and Upper MS Valley/OH Valley/Mid Atlantic/Northeast...

Short wave energy in the quasi zonal flow on the southern edge of
the westerlies will provide pockets of lift over a frontal
boundary that drops slowly south across the region during Days 2
and 3. The short wave energy interacts with deep moisture and
instability to produce convective clusters capable of producing
heavy to excessive rainfall, especially during Day 2 across the
northern Mid Atlantic states. There are still some model
differences concerning the placement of the front, especially
during Day 2. In an attempt to mitigate some of the remaining
differences, the WPC QPF was based on a multi model blend, with
more weight given to a blend of the 00z ECMWF/GFS.

Day 2...
Convective clusters should be ongoing early on Day 2 across
southern MI into northern NY state ahead of a frontal boundary
dropping south across central and southern New England. Short wave
energy in the mid level flow crosses the front as it drops slowly
south, reaching a position extending from eastern New England
across the NYC Metro area into lower OH Valley into the Mid MS
Valley before 16/00z. Ahead of the front, a low level south
southwest flow continues feeding 1.50/1.75 inch precipitable water
air (which is between two and three standard deviations above the
mean) across the northern Mid Atlantic (some of which is siphoned
off from a system over the Southeast states).

The best instability ahead of the front is expected here as well,
centered between 15/18z and 16/00z. Strong convection in the deep
moisture is expected to produce an axis of 1.00/1.50 inches of QPF
extending from southern OH into Southeast NY state, with the
highest amounts over southeast PA and nearby NJ. Three hour flash
flood guidance values are as low as 1.00/1.50 inches over portions
of northern WV/southwest PA and eastern PA and northern NJ. There
is some potential for training ahead of the front here, so a
Slight Risk area was extended over these areas. A Marginal Risk
was extended from southern New England into portions of the OH
Valley and Mid MS Valley as well, as both the 00z ECMWF/GFS showed
the potential for higher QPF amounts in these locations.

Day 3...
As the best mid level lift exits into eastern Canada during Day 3,
the frontal boundary slows across the OH/TN Valleys into the Mid
Atlantic states. A low level southwest flow continues to pump
1.50/1.75 inch precipitable water air into the Mid Atlantic and OH
Valley, with the source located over the Southeast states. With
the slowing front comes slower storm motions, and the combination
of high moisture content air and marginal to moderate instability
(locally higher over the OH Valley close to the front) could
result in cells producing local rainfall amounts in excess of 2.50
inches (as shown on the 00z GFS).

The highest rainfall amounts are expected over the Delmarva
Peninsula and southeast VA. A Slight Risk was placed here, as
recent heavy rainfall could make this area more susceptible to
flash flooding. A Marginal Risk was placed over much of the lower
OH Valley and Mid Atlantic, where slow storm motions and high
moisture content air could overcome relatively high flash flood


A closed mid level low over the eastern Gulf of Mexico at the
start of Day 2 moves slowly north, tracking into the Southeast
states later on Day 2 into Day 3. A surface low associated with
the mid level system approaches the central and eastern Gulf Coast
states during this period. Deep moisture and increasing low level
inflow will pose at least a low level flash flood threat each day.
There is better agreement among the 00z model suite members with
the placement of the mid level system, but there is some still
some spread on the forward speed of the surface system (with the
00z NAM faster than the consensus). The WPC QPF was based more
closely on the 00z ECMWF/GFS, which are close on the placement of
the surface low.

Day 2...
As the closed mid level low crosses the Gulf Coast and reaches the
Southeast states during Day 2, a low level southwest flow
continues to transports 1.75/2.00 inch precipitable water air
(which is between two and three standard deviations above the
mean) across much of FL, as well as GA/SC into NC. Model soundings
showed moderate instability over these areas, and the moisture and
instability is expected to feed mainly diurnal convection. Slow
cells motions associated with the mid level system could result in
local 3.00+ inch rainfall amounts over GA/SC/NC, and despite the
fairly high three hour flash flood guidance, may pose a flash
flood threat during Day 2. Thus, a large Marginal Risk was placed

Further south over the FL Panhandle and portions of southern AL, a
surface system associated with the mid level low meanders just
offshore. There is some spread on how fast the system moves
northward, but there is some signal that the low level southeast
flow ahead of it could begin to transports higher moisture to the
Gulf Coast. While the highest QPF totals were kept offshore during
Day 2, there could be enough convection across the Gulf Coast for
localized flash flooding issues, so a Marginal Risk was placed
here for Day 2.

Day 3...
The closed mid level low opens up into a broad long wave trough
over the TN Valley and Southeast states during Day 3. Moisture and
instability profiles are similar to those of Day 2, and slow cell
motions are expected (though they may be a bit faster than Day 2).
Once again, mainly diurnal convection is capable of local 3.00+
inch QPF amounts over the Southeast, so a Marginal Risk was placed
here for Day 3.

Perhaps the greatest flash flood threat on Day 3 will be
associated with the surface low as it comes ashore over either MS
or the FL Panhandle. As mentioned earlier, there are still some
differences with respect to timing/placement, so it is not clear
just how significant the flash flood threat will be. However,
there should be enough low level southeast flow (generally between
25/30 knots) to usher rainfall amounts between 2.00 and 3.00
inches here. The three hour flash flood guidance values are fairly
high here (mostly above 4.00 inches), so a Marginal Risk was
placed here on Day 3. If better timing can be achieved with the
surface low, this area could be upgraded to a Slight Risk, since
the potential for several inches of rain exists).


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


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