Post by rnj79: Addis, WBR Parish on Apr 11, 2020 9:00:52 GMT -6
Wasn't trying to bring any negativity. I'm tired of the local & world news and was looking for a way to escape all of that. This is a weather board and I was hoping folks would be talking about the weather. If it were a tropical threat looming, we wouldn't be able to keep up. I'll hush though. And yes, thank you, ThermalWind, for all of your detaailed explanations!
I'll definitely be enjoying Easter with my family tomorrow. Hope you all have a fantastic and blessed day. It's beautiful again this morning!
“On Christ, the solid Rock I stand. Everything else is just sinking sand.”
Post by Will - Ascension Parish on Apr 11, 2020 9:37:10 GMT -6
Yea, I don’t see any negativity here either. This forum is very pleasant and informative. Unlike some others. Thanks to everyone here who contributes and makes this place so great!
Post by mobileweatherwatcher on Apr 11, 2020 12:08:35 GMT -6
The NWS in Mobile has put out an statement on the severe threat. This is the first time they’ve identified an area that has the greatest potential
Area Forecast Discussion...UPDATED National Weather Service Mobile AL 1255 PM CDT Sat Apr 11 2020
.NEAR TERM /Now Through Sunday/...The bottom line during the period is that there is the potential for a significant outbreak of severe weather on Sunday especially north of the I-65 corridor but this is contingent on overcoming a strong cap centered near 850 mb. A cut off upper low located near southern Arizona/northwest Mexico transitions to a well defined shortwave trof while advancing eastward across the south central states on Sunday. A large northern stream upper trof meanwhile swings out of western Canada into the northwest states and northern/central Plains. In response to the shortwave trof and the northern stream system, a surface low develops today to the lee of the Rockies over the central Plains with a secondary surface low developing near southern Texas. Due to the interaction between the northern stream system and the shortwave trof, these two surface lows merge to form one large surface low centered over Oklahoma Sunday morning which progresses roughly along the Missouri/Arkansas border through Sunday afternoon. Ahead of the approaching surface low, a warm front advances northward across the north central Gulf and nears coastal portions of Alabama and the western Florida panhandle by daybreak on Sunday. The surface low brings a cold front eastward across Texas and much of Louisiana during the day on Sunday while the warm front continues northward through the forecast area. Dry and relatively cool temperatures over the area will be replaced with a moist and much warmer airmass in the wake of the warm front, with surface dewpoints increasing to around 70 to the lower 70s across much of the area except for values of 65 to 70 over portions of south central Alabama.
Shear values increase dramatically through the period, with 850 mb winds of 20 knots this evening increasing on Sunday to 50 to 55 knots, with even higher values of 60 to 70 knots indicated in the afternoon over portions of Wayne, Choctaw, northern Clarke and Wilcox counties. SBCAPE values increase to around 2000 J/kg by early Sunday afternoon across interior southeast Mississippi, southwestern Alabama and the far western portion of the western Florida panhandle while further to east values of 1000-1500 J/kg are expected. These CAPE values with high anticipated shear values result in potentially very high Significant Tornado Parameters (SigTor) of 6.0 to 16.0 north of the I-65 corridor, with the highest values seen over Wayne and Choctaw counties. High SigTor values of 4 to 8 are also seen east of I-65. These conditions are indicative of a potentially significant outbreak of severe weather across the area on Sunday, especially north of the I-65 corridor. That said, all of this is conditional on overcoming a strong cap seen near 850 mb, the strength of which varies from strongest in the NAM to becoming weaker and erodes in the afternoon per the GFS and HRRR. The potential for the cap to be overcome looks to be greatest north of I-65 and during the late afternoon hours on Sunday. This coincides with when a squall line is expected to have developed west of the area and will be nearing interior southeast Mississippi and interior southwest Alabama (Choctaw county).
The Storm Prediction Center has a Moderate Risk of severe weather across interior portions of southeast Mississippi and interior southwestern Alabama with an Enhanced Risk of severe weather over the remainder of the area. Would not be surprised is a High Risk of severe storms is ultimately issued within the Moderate Risk area. Stay tuned!!
Repeating this here from an earlier update: a Wind Advisory is in effect for the entire area from 10 am Sunday to 4 am Monday due to frequent gusts up to 45 mph. A Coastal Flood Advisory is in effect as minor coastal flooding is expected along coastal Alabama and coastal portions of the western Florida panhandle which is in effect from 10 am to 10 pm Sunday. A High Surf Warning is now in effect for coastal portions of Alabama and coastal portions of the western Florida panhandle for surf heights of 7 to 10 feet and is in effect from noon Sunday to 4 am Monday. /29
Post by thermalwind on Apr 11, 2020 19:19:58 GMT -6
Rolling along 287 up there in north Texas. Know every little town along that stretch. Watch where this resulting MCS ends up, as the farther north and west it ends up the bigger the warm sector will be tomorrow.
Something of note on the CAMs that has stood out today is how different the NAM has the surface conditions vs the HRRR and some others. I'm going to show two soundings for the same time at the same location.
Shear profiles are similar but look at the temperature at the surface. The NAMs have been keeping the cloud cover on, keeping the surface temps down. You get weaker lapse rates from the surface (closer to moist adiabatic with the saturation there) and maybe the boundary layer isn't really mixing? 1900hurricane (really smart dude) pointed this out on another forum this morning I think he has a point. You can see how the lowest wind barb on the NAM has way more of an east component to it, and how it doesn't line up quite as well with the strong winds around 850 vs the HRRR here. The idea here is the NAM isn't mixing the surface boundary layer out and all the storms are showing up elevated vs surface based, which is why the NAM is not showing much supercell type activity.
I'm not sure why it wouldn't mix with the wind and heat that will be pumping in from the gulf, but it's something I'm making a note of for watching things tomorrow.
Rolling along 287 up there in north Texas. Know every little town along that stretch. Watch where this resulting MCS ends up, as the farther north and west it ends up the bigger the warm sector will be tomorrow.
Something of note on the CAMs that has stood out today is how different the NAM has the surface conditions vs the HRRR and some others. I'm going to show two soundings for the same time at the same location.
Shear profiles are similar but look at the temperature at the surface. The NAMs have been keeping the cloud cover on, keeping the surface temps down. You get weaker lapse rates from the surface (closer to moist adiabatic with the saturation there) and maybe the boundary layer isn't really mixing? 1900hurricane (really smart dude) pointed this out on another forum this morning I think he has a point. You can see how the lowest wind barb on the NAM has way more of an east component to it, and how it doesn't line up quite as well with the strong winds around 850 vs the HRRR here. The idea here is the NAM isn't mixing the surface boundary layer out and all the storms are showing up elevated vs surface based, which is why the NAM is not showing much supercell type activity.
I'm not sure why it wouldn't mix with the wind and heat that will be pumping in from the gulf, but it's something I'm making a note of for watching things tomorrow.
Which model "normally" handles this type of stuff better? The NAM says "Tor" on Public Hazard, whereas, the HRRR has PDS. Is that for possible types of weather watches? I wish I knew how to read Soundings so I wouldn't have to ask so many questions!
Also, I know planes have not been able to do their usual tests of the upper levels of the atmosphere, do you think we could be missing some important information because of that?
“On Christ, the solid Rock I stand. Everything else is just sinking sand.”
Post by thermalwind on Apr 11, 2020 19:57:07 GMT -6
NAM tends to run cooler and HRRR warmer but there isn't a "better" really. It does tells us to watch those surface observations tomorrow and see how warm it's getting and what direction the wind is coming from.
NAM tends to run cooler and HRRR warmer but there isn't a "better" really. It does tells us to watch those surface observations tomorrow and see how warm it's getting and what direction the wind is coming from.
Got it. Thanks!
“On Christ, the solid Rock I stand. Everything else is just sinking sand.”
Post by mobileweatherwatcher on Apr 11, 2020 20:31:16 GMT -6
Kind of an interesting tweet from Reed Timmer here. Seems like this is one of those scenarios where the potential of an outbreak is high, but the potential for a bust is rather good too. Someone correct me if I’m wrong. Everyone should still be watching obviously in the southeast. But hopefully this is one event where the potential is not reached.
Post by thermalwind on Apr 11, 2020 20:38:48 GMT -6
He's right about the typical.
Plenty of moisture, plenty of shear, but lots of ways to fail. Cap comes off too easy and you get convection everywhere to interfere. Low level lapse rates weak and you get elevated storms vs surface based. Overnight convection leaving a cold pool behind and stopping the front from getting farther north.
With the CAMs as they've been showing, I'm guessing if there's a failure mode tomorrow it's the storms cant get rooted in the surface boundary layer and everything stays elevated.
Last Edit: Apr 11, 2020 20:50:21 GMT -6 by thermalwind
Post by mobileweatherwatcher on Apr 11, 2020 20:53:13 GMT -6
Reading from the SPC and discussions from the weather service offices in Mobile and New Orleans, there are still plenty of questions. My amateur opinion is if we get a high risk on the Day 1 outlook, it won’t be on the first update.
Post by thermalwind on Apr 11, 2020 21:01:46 GMT -6
My feeling is if we get high risk, which doesn't seem likely right now, it's going to be for the wind on the north side of the mod zone. Way too much uncertainty to pull the trigger on tornadoes.