Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
344 AM EDT Sat Jun 02 2018


Prelim Day 1 QPF Discussion
Valid Jun 02/1200 UTC thru Jun 03/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


...Central-Northern Plains and Upper Midwest...

Mid/upper level trough across the northern Plains early this
morning will become more negatively tilted during the day 1
period, as the more confluent upper flow across the central
Plains-mid MS Valley causes the southern periphery of the vort
lobe (trough base) to take off a bit faster compared to the
remainder of the trough. Upper level forcing will be bolstered by
the negative tilt (increased divergence aloft and more focused
low-mid layer Qs convergence), however the modest-at-best deep
layer instability will preclude the excessive rainfall threat to
some degree over areas that are expected to benefit from the
strongest dynamics (including MN-WI). Farther south, ongoing MCS
across the lower MO Valley will remain progressive early this
morning, the undergo decay after 12Z following the diminishing
nocturnal LLJ. Rainfall amounts into the mid MS Valley and western
OH Valley will be more modest on average, i.e. averaging under
1.0", though per the individual high-res CAMs, localized totals of
2-3+ inches will remain possible as the axis of favorable
thermodynamics (CAPES of at least 1000-2000 j/kg and anomalous PWs
of 1.5-1.75") along and ahead of the surface front pushes w-e
across the area. WPC QPF was based largely from the high-res
means, especially the HREF probability-matched mean in providing
more detail with the areas more likely to see heavier amounts.


...Lower Mississippi Valley...

Another more focused area of moderate-heavy rainfall will align
generally n-s/nne-ssw along the progressive cold front, with
vigorous deep-layer instability owing to the steep mid-level lapse
rates and PW values maxing out between 1.75-2.0". WPC also
utilized a blend of the high-res CAM guidance in this region with
the QPF, especially with the HREF mean, in generating
areal-average totals under 1.0" for most. Would also expect
localized totals of 2-3+ inches in this area as well; however,
given the swift progression (aided by the strong downdraft CAPE
and resultant convective outflows), the excessive rainfall threat
will remain somewhat mitigated (i.e. Marginal). 


...Eastern U.S....

Slow-moving upper trough will nudge eastward toward the Atlantic
coast today. Will continue to see little change in the overall
pattern until this deep, compact trough pushes off the coast.
Given the orientation, the models continue to indicate the more
focused area of forcing (DPVA, upper divergence) across the mid
Atlantic region during the peak heating hours this afternoon into
the evening. Thus expect more widespread convective coverage today
compared to yesterday across this region, where much rain has
fallen over the past several weeks and FFG values are quite low as
a result of the wet antecedent conditions. Light deep-layer
flow/low 0-6 km bulk shear (generally aob 10 kts) would again be a
huge deterrent for more organized, prolonged convection; however,
given the low FFG values (especially west of I-95), persistent
broad-scale forcing, ample/deep subtropical moisture, and
surface-based CAPES likely peaking between 1000-2000 j/kg prior to
the onset of convection, believe the hourly or even sub-hourly
rainfall rates will exceed the 1 hourly FFGs over many areas, as
depicted by the SSEO and HREF probabilities. Thus per
collaboration with WFOs AKQ, LWX, and PHI, have hoisted a Moderate
risk for flash flooding across a large portion of the mid
Atlantic, encompassed by Slight and Marginal areas.
In terms of the QPF, WPC again relied heavily on the high-res
means in this area, especially the HREF.

Hurley