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Isaac Heavy Rain Event


Hoosier

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But doesn't H5 just scream for this to get sheared off to the east, considering that block north of Isaac?

Not until it is inline with Detroit, from then on it looks like it would shear out. That block is pretty far East and doesn't drop in until late Sunday. All it will do is probably shaft your area and points NE from there.

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As far as moisture, it probably doesn't help that this thing will have been over land for a few days by the time it gets here, but I'm just not sure how much of a negative factor that will be. Consider the source region for this event. I'm not sure what the rainfall distribution will be like (typically it tends to shift toward the northern side of the track this far inland but maybe not in this case) but I'd bet on heavy swaths if rain even if it's not as uniform as the HPC graphic.

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One thing I have learned over the years - keep a very close eye on tropical remnants - even late into the night and early morning hours. We have had events in this region that produced 7-10" of rain in a matter of 3-4 hours. This from slow moving tropical rain producing cells.

Yeah, it seems like near the center is often the area to watch after dark. Can get some torrential rains.

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The 12z Euro has 2-4" for most of central/southern IL. One thing to keep in mind when coming up with amounts is that the precip process will be very efficient.

This and as Beau noted with remnant systems they tend to be very efficient. Which brings to mind, how many tropical system remnants over perform vs under perform for this region. Climo and past events lead way more toward over performing vs under performing.

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This and as Beau noted with remnant systems they tend to be very efficient. Which brings to mind, how many tropical system remnants over perform vs under perform for this region. Climo and past events lead way more toward over performing vs under performing.

It would be interesting to see how remnant systems have performed relative to models over the years. There's just not a ton of systems to go by...this isn't like tracking a winter storm or something that happens multiple times per year.

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18z GFS came NW again but this time in weakens it quicker as it moves northeast. Best rains from nrn/ne MO into western IL/southeast IA and into nrn IL.

The 18Z and 06Z GFS runs have been doing that for a while now...only to see the 00Z and 12Z runs correct back to the southeast. HPC forecast seems a little bullish but they are some of the vest forecasters around so we will see.

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The 18Z and 06Z GFS runs have been doing that for a while now...only to see the 00Z and 12Z runs correct back to the southeast. HPC forecast seems a little bullish but they are some of the vest forecasters around so we will see.

Doing a search of storms in the past 50 years, I can't find many examples of a storm producing the kind of widespread heavy/excessive amounts that the HPC graphic is showing. Carla and Frederic come close but different areas affected. It does seem to be uncommon and I was a bit surprised to see them being so bullish this far out.

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HPC QPF discussion for days 2 and 3, which takes us through 0z Sunday.

A NARROW RIBBON OF HIGH RAINFALL AMOUNTS WILL ACCOMPANY THE PATH

OF ISAAC AS IT SLOWLY WEAKENS WHILE ROTATING AROUND THE PERIPHERY

OF THE SUBTROPICAL RIDGE OVER THE SOUTHEAST THROUGH AT LEAST DAY

3/SAT...AND LIKELY BEYOND AS IT ENCOUNTERS THE APPALACHIANS. TO

ARRIVE AT THE FORECAST AMOUNTS...USED THE LATEST NHC FORECAST

TRACK AND THE ASSOCIATED DYNAMICAL AND TROPICAL MODEL SOLUTIONS

WHICH BEST ALIGNED WITH THE TRACK AS A BASELINE...THEN ADJUSTED

THE AMOUNTS UPWARD TO ACCOUNT FOR LOW QPF BIASES RESULTING FROM

INADEQUATE SPATIAL/VERTICAL RESOLUTION...TROPICAL OR WARM-RAIN

PROCESSES NOT ADEQUATELY PARAMETERIZED WITHIN THE MODELS...AND

NOCTURNAL ENHANCEMENT THAT OFTEN OCCURS AS SYSTEMS CONSOLIDATE AT

NIGHT. THE FINAL DAY 2 TOTALS...ENDING 00Z SATURDAY...ARE SIMILAR

TO THE NAMCONUSNEST AND HURRICANE WRF-ISSAC RUNS...THEN SLIGHTLY

DISPLACED TO ACCOUNT FOR THE ANTICIPATED TRACK. HOWEVER...EVEN

THESE TOTALS MAY BE UNDERDONE. THE TOTALS FOR DAY 3...ENDING 00Z

SUNDAY...WERE MORE DIFFICULT TO ARRIVE AT GIVEN THE INCREASING

MODEL SPREAD. HOWEVER...THE GFS/ECMWF/UKMET WERE REASONABLY CLOSE

TO THE NHC TRACK TO USE AS A PARTIAL GUIDE...BUT AGAIN THE QPF

AMOUNTS ARE LIKELY TOO LOW DUE TO THE ABOVE MENTIONED REASONS.

ADDITIONAL FACTORS CONTRIBUTING TO HIGHER QPF FOR DAY 3 INCLUDE

INCREASING INTERACTION WITH THE NORTHERN STREAM CROSSING THE GREAT

LAKES...WHICH FACILITATES ELONGATION OF A ZONE OF LOWER-LEVEL

CONFLUENCE AHEAD OF ISSAC AND DOWNSTREAM TRANSPORT OF DEEP PW AIR

THAT ASCENDS WITHIN THIS ZONE. OVERALL CONFIDENCE IS ABOUT

AVERAGE ON DAY 2...BUT THEN DECREASES TO LESS THEN AVERAGE ON DAY

3 AS THE SOLUTION ENVELOPE WIDENS.

Day 3 map below.

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