Mesoscale Precipitation Discussion 1062
NWS Weather Prediction Center College Park MD
949 AM EST Fri Dec 21 2018

Areas affected...Portions of the Northeast

Concerning...Heavy rainfall...Flash flooding possible

Valid 211447Z - 212030Z

Summary...Unusually significant amounts of moisture for the winter
season will continue to surge into the Northeast region today,
leading to widespread moderate to heavy rain. Areas of flooding
will generally become more likely through the day, with numerous
locations expected to receive 1-2 inches of rain in the next six
hours (by 3 PM). Isolated locations could receive up to 3 inches
of rain in that time.

Discussion...Any time you see monthly precipitable water records
falling, there tends to be an elevated risk of flooding issues.
That is the expectation across portions of the Northeast today, as
both Upton, NY and Albany, NY reached their December monthly
record PW on their 12Z soundings. Additional sites are likely to
eclipse their monthly record PW today, even if it doesn't occur
precisely at the subsequent 00Z observation time. The tremendous
northward moisture flux is the result of a ribbon of strong
southerly low-mid level flow to the east of an amplified trough in
the Eastern CONUS, and MIMIC-TPW and GOES-16 Simple WV RGB
products show that the tail of this moisture plume extends down
through Honduras and into the tropics. Not only are PW values
eclipsing monthly records, but the 925-500mb temperatures are all
above the 99th percentile for the month as well. Basically the
entire sounding profile is shifted to the right side, and in some
cases outside, of the December sample across the entire region,
and this will prove to be supportive of very heavy rain for this
time of year. One last data point for context: the RAP is
forecasting PW values around 1.5" at Gray, ME (KGYX) by 00Z, which
would be around the 90th percentile for that location during the
summer months.

The flooding concern across the region is essentially split into
two primary areas. First, greater instability will exist in the
lower elevation areas of southern New England, generally along and
south of I-90. Although MUCAPE should generally only be on the
order of several hundred j/kg, given the record PWs and strong
low-level convergence it should be sufficient for some convective
banding and perhaps a few areas where rain rates can approach 1
in/hr. This may be sufficient to cause flash flooding,
particularly where the heavier rain rates can intersect with urban
areas. Otherwise, this area can expect persistent moderate to
occasionally heavy rain through the day which should make general
areal flooding more likely. The second area of concern extends
further north in New England where there is actually still some
snow cover. This is primarily confined to ME, NH, VT and the
Berkshires in W MA. NOHRSC analysis shows over 2 inches of snow
water equivalent still in the snow pack at the higher elevations
(generally above 1000 feet). Given the anomalous warmth through
most of the troposphere, and the expectations for rain falling
through the day, additional runoff through snow melt should be a
consideration. Therefore, although rain rates will not be as high
in these areas, combined rain water and snow melt runoff should
lead to some flooding issues developing today.

Lamers

ATTN...WFO...ALY...BGM...BOX...BTV...CAR...GYX...OKX...PHI...

ATTN...RFC...MARFC...NERFC...

LAT...LON   45616875 45356736 44656688 43916865 43567000
            42787112 41817162 40887220 40227342 39367409
            39927448 40937460 41957428 43537325 44537212
            45447039