Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
254 PM EDT Sat Apr 14 2018


Prelim Day 1 QPF Discussion
Valid Apr 15/0000 UTC thru Apr 16/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


...Lower MS Valley into TN and OH Valley and the Southeast...

Multi-cellular linear convection remains well organized (and
progressive) early this afternoon across the lower MS Valley and
western TN Valley ahead of the cold front. Deep-layer,
surface-based instability downwind remains favorable for a
continued progressive squall line, with MUCAPES between 1000-2000
j/kg along with modest downdraft CAPE (400-800 j/kg). Continue to
expect some slowing in terms of the eastward progression of the
convection this evening, owing to the arrival of shortwave energy
and resultant upper height falls across the western Gulf region,
thereby allowing for a more negative tilt in the longwave trough
orientation. In addition, the nocturnal uptick in the
above-surface low-level flow this evening and overnight (50-55 kts
across the eastern TN Valley by 04Z) will also aid in decelerating
the forward (downwind) propagation, especially after midnight as
the LLJ orients more parallel to the mean 850-300 mb flow. Thus as
with last night across portions of the lower MS Valley (central AR
in particular), expect a heightened risk for training/repetitive
convection across portions of the TN Valley (AL into eastern TN),
as moisture parameters remain impressive with strong 850 mb
moisture transport and PWATs approaching climatological max
values. The one limiting factor in terms of the flash flood threat
would be the anticipated rain rates/amounts -- both of which would
likely be curtailed by the more limited instability (500 mb j/kg
or less).

WPC QPF for the warm sector of this system generally blended the
HRW ARW, ARW2 and the HREF Mean. The HRRR was also heavily used
early in the period, as it was the best match of the high res
guidance to the current radar. The ARW was also a good match, as
the previous (00Z) version had become a bit too progressive by
this afternoon. Given that organized convection is ongoing and
will maintain through the day, generally stuck with the high res
guidance for this warm sector QPF, as it is situations such as
this when they typically are much more useful than the global
guidance.


...Upper MS Valley---Great Lakes into northern NY and northern New
England...

A surface low across the central Plains-mid MS Valley will track
east into the OH Valley tonight. To the north and northwest of
this low will see continued wintry precipitation across
MN/IA/WI/MI and far northern IL/IN. Strong 850 mb moisture
transport ahead of the low will transport convection up and over
the warm front into MI tonight, although rainfall intensity north
of the warm front should be tempered by the lack of instability.

In general, models are in pretty good agreement, with the latest
WPCQPF exhibiting good continuity from our previous forecast
(especially along the TROWAL).

Hurley/Chenard