Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
203 AM EST Sat Mar 10 2018


Prelim Day 1 QPF Discussion
Valid Mar 10/1200 UTC thru Mar 11/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

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

A heavy rainfall event still appears likely Saturday evening into
the overnight period across portions of AR/MS/AL, although still
some lingering uncertainties with regards to the exact axis and
magnitude of highest totals. Models are however in much better
agreement than they have been...and was thus able to add in some
more detail to the QPF forecast than our previous issuance.
Showers will continue this morning into the afternoon from the
central Gulf Coast northeast into the TN valley associated with a
weak lead wave...with this initial activity generally being light
in nature given limited instability.

By around 0z this evening will begin to see better mid/upper level
forcing move back into the lower MS valley. In response should see
an uptick in 850 mb moisture transport and a tightening of the low
level west to east stationary boundary. The combination of this
increased synoptic support and low level moisture convergence
should result in an expansion of convective activity by this
evening across AR/MS/AL along/near the aforementioned boundary.
Models all seem to be in agreement with this evolution occurring.

PWAT values are only marginally above average...and not really at
the magnitude you would like to see for efficient heavy rainfall.
Although the values approaching 1.2" would still support the
potential for some 1-2" an hour rainfall rates. 850 mb moisture
transport is also not as strong as you would normally see for
excessively heavy QPF amounts. Instability is not forecast to be
that high...although does look we initially see enough surface
based instability over AR for development...and enough elevated
instability further east to sustain convection into MS/AL. Thus
there will be enough increased mid/upper level forcing and
instability present to allow for the development of robust
convection by this evening. While the magnitude of 850 mb moisture
transport is not great (as mentioned above)...the corridor of
enhanced transport is broad. This thus supports an elongated axis
of moisture convergence...which should be supportive of some west
to east training and repeat convection. It is this factor that
will likely result in a corridor of 2-3" (locally higher) of
rainfall and the development of at least some flash flood threat
into tonight (despite the marginal pwats/instability).

As mentioned above, models are in better agreement with this
convection and thus QPF totals through the day 1 period. Our axis
of 1-3" stretches from central AR east into portions of
central/northern MS/AL (highest in MS/AL where some training is
most likely)...and this is the consensus location amongst the
latest global and high res models. As far as the magnitude
goes...did lean more towards the high res guidance...which were
consistently higher than the global guidance. This peaks amounts
in the 2-3" range (although locally could see upwards of 4").
Amounts much higher than this seem unlikely at this point given
marginal PWATs/moisture transport and the progressive nature of
the system.

Overall our axis of higher totals is pretty similar to our
previous forecast. Although this forecast does cut back quite a
bit on the extent of the 1" area. Some of the previously broader
models with the QPF have since backed off...and it is becoming
clearer that the heavier totals should stay confined to a narrower
axis near the low level convergence axis. 


...South Florida...

The presence of a mid level wave and the development of impressive
upper level divergence will result in widespread convective
development over the Gulf of Mexico today. This activity will move
east towards central and southern Florida through the day. In
general think the activity will be in a weakening state as it
moves ashore given limited instability. However appears
increasingly likely that at least some steady showers...and
possibly a few embedded heavier convective cores will move across
southern Florida this afternoon and evening. Will need to monitor
trends through the day...as some signs in a few CAMs that some
heavier convection could impact far southern Florida and the Keys
by this evening as some instability tries to work its way north.

WPC followed a multi model blend for QPF across this
region...which is a decent increase from our previous forecast.
For now kept areal averaged amounts below 1"...but as mentioned
above will need to monitor trends as some chance we see locally
higher totals.


...Southwest...

A wave moving into the southwest will bring a broad area of
precipitation into southern CA and inland into the remainder of
the southwest through the day 1 period. An impressive plume of
moisture is noted...with PWAT values pretty much as high as you
can get this time of year over the southwest. This combined with
the wave/upper jet certainly supports an expansive area of light
to moderate precipitation mounts. Limiting factors will be the
flatness of the wave...which is not really supportive of heaver
rates...and also low level wind fields that are not all that
anomalous. So while we certainly will see some orographic
enhancement over southern CA...not expecting rates to be that
impressive. Current indications are for rainfall rates to
generally peak within the 0.25"-0.35" range within an hour across
portions of the southern CA terrain this afternoon and evening.

Models have trended further south with the wave and quicker. This
results in a shift south in the heavier QPF across CA and a
further inland extent of precipitation across NV/AZ/NM into
tonight. Expecting generally between 0.5"-1" across much of
coastal southern CA...with 1-3" possible in the more favored
terrain. 

Chenard