All types of rain amounts being thrown out there ...so I know it is confusing on what to expect this weekend and when.
Let me break it down to you this way.
Here is what we DO know:
-We do know the low (Isaac) will track north toward Missouri
- We do know we are going to be on the east and/or southeast side as the low moves through
- We do know rain is likely Saturday/Sunday and Monday
- We do know someone is going to get several inches of rain
- We do know the rain will come down extremely hard...making travel difficult
- We do know it will be breezy at times.
What we DO NOT know:
- We do not know the exact path of the low pressure after it arrives in Missouri
- We do not know the forward motion speed of the low as it moves through
- We do not know where the strongest bands of rain will develop.
- We do not know if training of the storms is going to take place
- We do not know if we will see severe storms.
- We do not know the specific times of day in which rain is more likely over another.
Decaying tropical systems like this are a challenge as the westerly wind belt that moves over our latitudes can help keep the low energized enough to produce some gusty winds/heavy rain. That can sometimes go against want a model indicates. The key to these events is to see how it has "behaved" compared to the models thus far. One thing that has stood out there---rain amounts have been exceeded model amounts.
I do think that is why HPC is going pretty high on forecast amounts.
Because their amounts are certainly higher than the model outputs.
Model Maps:
GFS:
NAM:
Specific Output numbers for Louisville:
GFS: 1.11"
NAM: .94" * can only see through Sunday evening
EURO: 3.10"
HPC will update their map in about 90 mins. We will post it once it is issued.
Right now, I think 1-3" with pockets up to 4" is certainly feasible.
If dewpoints do climb higher than the low 70s current forecast---yes, I could see the 4-7" rain amounts becoming more realistic. Especially with training of thunderstorms over the same spots..over and over again. I just do not see that being widespread right now and we don't want to cause a panic over such high rain totals.
We are still 48 hours away from the heaviest rain moving in---so we have time to fine-tune and adjust. Just keep checking back.
WINDS:
The winds will not be anything like what the Gulf witnessed---but gusts of 30 to 35 mph are possible.
NAM can see out to Sunday---and it does have some elevated winds aloft---but nothing alarming.
GFS is a bit more aggressive late Sunday with a 40 kt wind right on the SE corner of the low. We will have to watch to see if that comes to light. That, combined with thunderstorms, could lead to a damaging wind threat Sunday evening.
SPC does not have us outlooked for severe weather yet. They seem to be taking their outlooks day by day. This tells me they have low confidence on the track and instability as well. I would agree.
EHI scans on the NAM for Sunday are off the chart. I would expected tornado warnings potentially in that area.
NAM cannot see what happens after that...but if it follows what the GFS was hinting to---some brief rotating storms may be possible later Sunday into Monday. We'll watch that.
TIMING:
Clouds from Isaac already teasing the KY/TN border today. We will see those stream north tonight/Friday.
Friday- chance looks to be about 20% and mostly where the dewpoint surge will be. Which would take place along and south of a line from near Evansville to Greensburg.
Friday night- showers continue in the same zone above..but lifting closer to the Ohio River by Saturday morning.
Saturday. chance looks to be about 60%. We will start to see the eastern-most bands from Isaac affecting the area. Storms will be spotty...but producing heavy downpours. up to 1 of rain is possible for some by Saturday evening.
Saturday night: chance should increase to 70%. Rain remains on the radar. Locally heavy.
Sunday: Chance at 80 to 90%. Periods of rain/thunderstorms. If the main low is across IL/IN...that is where the steady hours on end rainfall take place. For our viewing area--it will likely fall in waves. Each one with very heavy rain. Severe threat would reach its peak at this point as well as gusty winds alone from the low pressure. Sunday looks to be the main day for Isaac to affect us full on.
Monday/Labor Day. The low begins to lose its characteristics as a low pressure and becomes more frontal in nature. So the winds will relax. But bands of storms will continue. Highest chances look to be east of I 65 by Monday afternoon. NOTE: the latest EURO aims at MONDAY as being the heaviest rain day over Sunday. That is a new trend on this latest run. I would like to see the run tonight match that before jumping on it.
We are not fullly there yet folks with all the answers...but we are getting closer and closer :)
By the way--- there is a decent trough/cool down showing up near the 8th. I havne't forgot about long-term tracking as well. Just the above event is more pressing right now.
The HPC has been pretty aggressive about rainfall amounts. I know the rainfall amounts have exceeded model guidance. However, historically, tropical systems tend to weaken and lose a lot of their initial moisture feed.
Therefore, I expect the HPC to show robust amounts for the region; however, we should remember that those maps only show the POTENTIAL (caps for emphasis) for SOME areas in specific shaded regions to receive those amounts. Not everyone will see those totals. Most areas should average 1-3", somewhat higher in 'training' bands.
**** WWW -- that is why we are going with 1-3 with 4" amounts possbile. - bg
Posted by: Www | August 30, 2012 at 04:02 PM