As we move deeper into April, the threat for weather continues this week with several days presenting the opportunity for severe storms and one day in particular, Thursday, appearing the day with the highest risk. Rainfall will again be heavy, especially in the east, where rains have been plentiful, with less significant, but still notable rains in the west where dryness and drought are an increasing concern.
Let’s take a look at the upper air pattern as we move through the week.
Notice the jet stream is retreating north with the polar lows now rotating from Quebec to the Davis Strait and over the Canadian Archipelago. An upper trough will track through the western and central U.S, this week providing the dynamics and lift for thunderstorms. Another trough begins to take shape NEXT weekend.
Midweek (Tue-Thu) is the prime concern. A Marginal risk exists and extends into the far southern portion of the western district Tuesday, while an elevated risk develops over the entire west and areas NW of STL in the east Wednesday. Tuesday will see a weak cool front settle south into central MO/SE KS with the marginal risk along and south of that boundary. Wednesday, that front turns around as a warm front and lifts north and westward, with the threat along and west/north of the warm front.
The main reason severe weather is not a slam dunk Wednesday is due to the presence of an Elevated Mixed Layer (EML) or a “cap”. This is something you’ll hear a lot about from now until the cool season returns. The EML/cap is a hot and dry air layer that forms over the hot Mexican Plateau and Texas Big Bend Country. This layer is pulled northeast by winds aloft. Since the air is warm and dry, it does not sink, but moves vertically as the terrain drops out from below. Below the EML/cap a thin, but increasingly deep layer of warm moist air is found. Where the EML/cap meets/leaves the surface is called the “dryline”. East of the dryline, its warm, humid and frequently cloudy. West of the dryline it can be hot, clear, dusty and dry. The dryline can act as a “front” forcing warm moist air (where deep enough) up along it’s edge into strong to severe storms. These dryline storms out in KS/OK/NE are frequently the ones you see the storm chasers snapping spectacular photos of. For us, we’ll be dealing with the northern edge of the EML/cap aloft, which will be south of the surface warm front. The warm front itself provides the “lift” needed to generate storms and rain. South of the front, there can be nothing to assist the “lift” needed.
This week, the air mass south of the warm front and well east of the dryline (our region) will be warm and humid, but the EML/cap may be too strong to allow the warm moist air to break the “lid” established by the EML/cap. What will be needed is a wave aloft to pull upward on the warm layer to cool/weaken it aloft or a complex of storms to generate a strong cool outflow boundary (a weak mini-“front”) to produce the extra lift from the ground to break the cap. Typically the cap is strongest west/south and weaker north/east and the western district is more at the mercy of this process.
Severe weather on Wednesday is rated lower in probability as it is more conditional, based on how strong the cap may be, where the warm front eventually winds up, how cloudy the warm sector is and the strength of any incoming waves aloft or where thunderstorm outflow boundaries are at the surface.
Thursday’s severe outlook is higher and more certain due to the fact the main system aloft and cold front are expected to move through. Some sort of squall line looks likely, but there are still timing issues as to when the cold front moves through, for example. Currently, the eastern district is more in line for a greater severe threat because the storms look to develop near or on top of the western district and then intensify/spread eastward. Should this scenario be slowed 6 hours, that threat might be shifted west to center on the KC area, while should it be sped up 6 hours, the west might be cleared from the risk while the storms form over the STL area and expand/intensify into IL. Four days out, and a lot can and will change; though compared to yesterday, there has been a slight slowing/westward shift.
The main thing to keep in mind is there is a likelihood of severe weather somewhere in/near the district this week. Right now the east has a better overall probability of seeing severe weather and rain as this could be one of those situations where the west misses the action, storms form south of the area Tuesday, too capped to produce any rain/storms Wednesday and storms form east of the area Thursday. That’s a a frequent pattern whereby an active wet week turns into a damp, quiet, cloudy week with lots of drizzle. The bottom line is everyone-west and east-need to be severe weather aware this week just in case, its too early to rule out any possible scenario.
Subtropical high pressure is becoming established over the area around southern Mexico. That high will build north to the latitude of Baja California/northern Mexico/the Gulf of Mexico and Florida in the next 4 to 5 weeks. in 6-10 weeks time, that high becomes the summertime ‘heatwave’ high pressure dome which dictates the thunderstorm track for us here in the Midwest/eastern Plains. It typically centers over the hot Desert Southwest and them moves to near the 4-corners in August. That’s a good position as it directs nightly storm clusters forming off the High Plains/Front Range southeastward into our region giving (the western district) most of it’s yearly rains.
If drought persists in the central/southern Plains, that area may become “hottest” and the high may center further east/north over the Panhandles/OK or Kansas. That would be bad news for us as it would spell a hot and mostly dry summer as rain would be directed well north and east of the region.
That’s why it’s important we get some rain to the drought areas in the next 1-2 months. As of the last update last week, dry conditions are established from KC west and south with drought as close as Emporia KS. Drought worsens as you head southwest from there and deeper into KS/OK/TX. The rains of April 1/2 are not included in this graphic and they may be enough to keep the dryness held in check another week, but we’ll need a SURPLUS of rain in the weeks to come to really beat back the edges of the drought and also to reduce the intensity at the core of the southern Plains drought region.
This next week’s precipitation outlook is not all that promising with 7-day rains of less than 1″ over the dry/drought areas and totals of 1/4″ to 1/2″ for the core of the drought zone. Some of this is related to the more scattered/conditional rains discussed above in the severe weather section. Eastern areas, which have had plenty (and in some cases too much) of rain continue to get soaked with 1.5″ to 3″ of rainfall. Flooding is becoming a concern here.
The longer range CFS model holds out hope for a wetter last three weeks of April and a wetter first three weeks of May, blues and purples are surplus rains while greens/browns and reds are deficits. The sometimes too optimistic CFS targets the core of the drought areas from CA to KS/OK with heavy rains between now and May 20th.
Week In Review:
Despite the late week chill, April has been running mild over all of the US except the NW and NE corners:
Springtime rain and thunderstorms were the main theme, with rains arriving in the west and strong thunderstorms in the east. Colder weather rounded out the week with frosty weather west Saturday morning and the threat of frost east Sunday morning.
On the the morning of April 2, showers and thunderstorms arrived in the KC area with some much needed rainfall. There were no issues with severe weather.
Redevelopment of isolated thunderstorms that evening brought a very strong storm through the SE Missouri Ozarks with 3″ hail near Potosi, and a warning being issued for St. Francois County, MO.
The most severe weather was focused well west and south of the region with reported tornadoes in SW MO/SE KS. This radar image shows a storm with a reported tornado northwest of Joplin, MO. Notice the distinct hook echo look to the standard image. The right side image is a doppler storm relative velocity image. It shows inbound (toward the radar) winds in blue and outbound (away from the radar) winds in red. The area where the deep blues and deep reds are close shows strong rotation, and it’s very close to the “hook”. Remember even this close to the radar, the beam is scanning in the lower cloud layer, so it doesn’t always mean a tornado is on the ground –but in this case storm spotters and law enforcement had confirmed a tornado on the ground. Tornado warnings are issued when storms show rotational signatures as those signatures mean tornadoes can occur at any time (warnings may state “radar indicated tornado”) so always remember to take cover if you are in a tornado warning – whether or not an actual tornado has been observed. Often times tornadoes begin away from areas where they can be observed and by the time they are observed, it may already be to late to take cover if you’re in the path.
The other story was the rain. Widespread significant rains fell, the heaviest in the east:
As discussed last week, warm season rains are much less uniform in nature and it is clear to see localized bands of heavy rains and localized areas which managed to miss the tracks of showers and storms.
The storm was followed by cold weather late week with Saturday morning lows in the 20s away from major metropolitan areas in the west.
Frost Advisories were issued Sat Night/Sun for parts of SEMO/S IL, but an increase in high clouds kept temperatures a little warmer.
Last week we discussed Super Typhoon Maysak (04W) which was quite strong over the southern reaches of the NW Pacific. Here are a few images taken from the International Space Station as it flew over the Super Typhoon.
Maysak (04W) weakened dramatically during the week and made landfall over Luzon, Philippines as a weak tropical storm. It will now slowly dissipate over the South China Sea as it gets caught up in cold northeasterly winds blowing off mainland China.
Another system, a weak tropical storm (Haishen/05W), formed near the equator in the NW Pacific, but did not persist long, due to strong wind shear aloft. The satellite image below shows one lone thunderstorm complex over the eastern side of the center. The rest of the system was a swirl of low and mid level clouds.
It is now meteorological autumn in the southern Hemisphere and we’re passing peak season for tropical cyclones in that hemisphere. There have been none this week but the southern Indian Ocean has two potential systems which may flare this week. Both are in the SW portion of the basin and well away from land. Tropical Disturbance 13 and Tropical Depression 14. TD 14 is moving southeast toward the Australian zone and if it reaches tropical storm intensity before crossing it would be named “Ikola” a name submitted by Tanzania. TD 13 will be named “Joalane” . Neither system is expected to threaten land, although TD13 will be watched for any impacts on the Reunion Islands east of Madagascar.
*Update: TD 14 was upgraded to Moderate Tropical Storm Ikola by RMSC Reunion at 1 PM Sunday, so TD13 would carry the name “Joalane” should it develop further. The U.S. Joint Typhoon Warning Center does not issue advisories or number tropical cyclones below tropical-storm force in the SW Indian Ocean and Ikola was still designated investigation area “93S” with a medium chance at development at the the time of this writing. Ikola would be designated “21S” unless JTWC began issuing advisories on TD13 (Investigation area “91S”) first, in which case it would be numbered “22S”.