Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Mon Apr 02 2018


Prelim Day 1 QPF Discussion
Valid Apr 03/0000 UTC thru Apr 04/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

Northern Plains---Upper MS Valley---Great Lakes---Ohio Valley

As deep low pressure remains firmly entrenched across Hudson Bay,
a vigorous shortwave over the northern intermountain west early
this afternoon will push ese into the mid-upper MS Valley by late
Tuesday. Ensuing lee-side cyclogenesis across the Central
Plains-Mid MS Valley-Ohio Valley during the same time will allow
for an expanding baroclinic leaf/area of deepening moist
isentropic ascent. Dynamical support will be optimized across the
OH Valley -- given the coupled favorable forcing via the
approaching shortwave and from the right entrance region of the
upper jet streak to the north (owing to the strengthened upper
level confluence south of the Hudson Bay low). Not coincidentally,
low-mid layer moisture transport is highest over this region per
the guidance, evidenced by the 850-700 mb moisture flux anomalies
between 4-5 standard deviations above normal per the latest GEFS.
Aiding to the favorable thermodynamical profile is the relatively
steep lapse rates aloft (nearing 7 c/km between 850-500 mb ahead
of the surface warm front and dry slot/warm sector south),
resulting in modest elevated instability (400-800 mb mucapes)
distributed in a relatively deep layer between 06-12Z Tuesday.

In terms of the QPF, the WPCQPF was decided wetter over the
previous forecast along and north of the I-64 corridor between STL
and CVG, owing to the high-res CAMs (WRF ARW, ARW2, and NAM CONUS
nest in particular) that along with recent HRRR runs, appear to be
doing a better job capturing the latest short-term thermodynamic
trends. WPC also hoisted a MARGINAL risk area in an updated Day 1
excessive rainfall outlook (ERO), based on the aforementioned
thermodynamic factors noted above along with the antecedent wet
soils/relatively low FFG values. In addition, within the MARGINAL
risk area, the wsw low-level inflow increases to 50-55 kts at 850
mb overnight, i.e. approaching the magnitude of (while aligning
parallel to) the mean 850-300 mb flow. This will favor increased
upwind propagation and cell training/repetitive convection
overnight into early Tuesday, especially between 06-12Z as per the
high-res hourly and 3-hourly rainfall rates. The risk area
encompasses the highest HREF probabilities of 3 hourly rainfall
totals exceeding 1 inch (40-60%). 

Farther north, WPC noted another QPF maxima (of lower
liquid-equivalent totals) between 0.50-0.75+ inch across portions
of the upper Midwest (southern MN, much of WI, and into northern
Lower MI) -- associated with a robust cold conveyor belt (ccb)
that will produce a swath of late-season heavy snow. For further
details, please refer to the latest QPFHSD.

Hurley