Nevada, IA Marine Weather and Tide Forecast
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Marine Weather and Tide Forecast for Nevada, IA

June 22, 2024 2:06 AM CDT (07:06 UTC) Change Location
Sunrise 5:37 AM   Sunset 8:55 PM
Moonrise 9:11 PM   Moonset 4:42 AM 
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Area Discussion for - Des Moines, IA
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FXUS63 KDMX 220353 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 1053 PM CDT Fri Jun 21 2024


Updated for the 06z Aviation Discussion

KEY MESSAGES

- Showers and thunderstorms in northern Iowa tonight. Heavy rainfall and flash flooding likely, with the chance for a few strong to severe storms capable of producing small hail, damaging winds, and a few brief tornadoes.

- River flooding expected for multiple area rivers in response to heavy rainfall tonight. See hydro discussion below.

- Additional severe chances expected tomorrow afternoon, with wind and tornadoes being the primary threat, although some small hail is possible.

DISCUSSION
Issued at 303 PM CDT Fri Jun 21 2024

Synoptic Overview

The frontal boundary that brought heavy rainfall and flash flooding to the far northwest portions of Iowa last night remains in place near the Iowa/Minnesota border and will again be the focal point for the weather over the next 12-18 hours. This boundary is one part of a larger synoptic pattern which has a surface low to our west slowly tracking north and east along with some weak upper level troughing.
As this surface low approaches the state, an increase in forcing and moisture will provide fuel for showers and thunderstorms across the northern portions of the state, resulting in a high likelihood of locally heavy rainfall and a chance for a few strong to severe storms.

Heavy Rainfall Tonight

The much advertised heavy rain threat continues to trend south into Iowa with the most recent guidance. Convection overnight last night has brought the surface front slightly further south than expected, which has pushed the most likely axis for heavy rainfall further south as well. As has been mentioned in previous discussions, convection will initially develop along the border but will start to leak south with the thunderstorm outflow and new development favors the better, less worked over air to the south. However, at the same time the low level jet will begin to oscillate east and increase in strength, which will eventually hold new development along wherever the surface boundary sets up. This, in conjunction with slow storm motions to the east, will result in storms training over the same general areas through much of the night, up until the shortwave brings an MCS along the boundary and washes out any new development tomorrow morning. This will ultimately result in a prolonged period of rainfall roughly along and south of the Iowa/Minnesota border, which becomes problematic given how moist the environment is. Strong moisture transport will push PWAT values over 2” in northern Iowa (almost 200% of the normal value for this time of the year) making for very efficient rainfall. As a result, expecting rainfall amounts of 2 to 4” to be common in northern Iowa with a high likelihood for a band of locally higher amounts pushing 7 to 8” or more. With rain amounts of this magnitude, flash flooding will be likely, especially in urban areas where drainage is poor, and significant flooding is forecasted for multiple area rivers.

While the conceptual model certainly checks out for heavy rainfall this afternoon, the exact location for the heaviest rainfall is still not trivial. As alluded to earlier, the heaviest rainfall will be where the front stalls, which will be strongly influenced by thunderstorm outflow and how convection plays out this afternoon and evening. It may also be augmented by the synoptic evolution of the surface low and LLJ, which could lift things further north. That all being said, convection has already begun along the Iowa/Minnesota border, which will start the process of pushing the boundary south and will favor the southern solution for heaviest rainfall. Models have started to key onto this trend and have started to place the heaviest rainfall over our northern two tiers of counties.
Therefore, to account for this continued southerly shift, have also expanded the flood watch down to the Highway 20 corridor to allow for some buffer in case the models continue with their northern bias and mesoscale processes end up pushing precipitation further south.

Severe Chances Tonight

Not to be overshadowed by the heavy rainfall and flash flood chances, there is also a chance for a few strong to severe storms this evening. Instability on the warm side of the boundary will be in the 2000 to 3000 J/kg range with shear values in the 20 to 30 kt range, which would support a few organized storms and a multicellular storm mode. Similarly, there will be vorticity generated along the surface boundary with 0-3 km CAPE values in the 100 to 200 J/kg range and LCLs basically on the ground, which would promote some brief spin up tornadoes as the vorticity is stretched in the vertical. DCAPE values aren’t overly high near the convection, but wind could become a factor as storms grow upscale into an MCS overnight and begin to ingest the surface boundary, leading to locally enhanced winds. That all being said, there are also multiple limiting factors, including warm profiles, marginal shear, and poor mid level lapse rates. Likewise, heavy rainfall will work to stabilize the boundary layer, further limiting any surface based hazards later in the night. Therefore, with weak shear and warm cloud processes, large hail is unlikely, but the environment will still be supportive of some isolated strong wind gusts and even a brief tornado. The slight risk for severe weather from SPC remains in generally the same area as this time yesterday, with just a slight expansion further south to account for the southerly trend in guidance.

Severe Chances Saturday

The fun doesn’t quite end on Saturday morning, as the surface low will pass overhead and bring it’s cold front through the area.
Fortunately, at this point the system will be more progressive, which should help to mitigate flash flooding concerns despite the heavy rainfall caused by moisture pooling ahead of the front.
Although rainfall will be less of a concern, the severe environment does look a bit better. Instability values will be similar to today’s with 2000 to 3000 J/kg of SBCAPE in the afternoon, as well as a bit more deep layer shear of 30 to 40 kts, which will favor a few organized storms. Warm cloud processes will likely still negate hail chances, but high DCAPE values in the afternoon will make wind a threat with any stronger storms. There will also be marginal amounts of low level shear and increasing low level lapse rates as the surface warms in the afternoon, suggesting tornadoes could be possible, although the better threat will be nearer to the low northeast of our forecast area. The day 2 outlook from SPC has upgraded us to a slight risk for much of eastern Iowa.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/
Issued at 1053 PM CDT Fri Jun 21 2024

Main concern contiinues to be storms building over northern Iowa affecting mainly FOD/MCW/ALO with potential for IFR cigs and low vsby during periods of very heavy rainfall. Have not modeled in wind gusts, but there continues to be a slight potential for brief strong wind gusts in periods of heavier storms between 06-12z at those locations. Farther south, much lesser impacts other than an isolated storm or two for DSM/OTM during the overnight/early morning hours. There is increasing confidence that we will see another round of storms 18z Sat to 02z Sunday as the main cold front sweeps southeastward across the state, but confidence in the timing of storm impacts is too low at this point to include anything more then SHRA/VCTS in the TAFs.

DMX WATCHES/WARNINGS/ADVISORIES
Flood Watch through Saturday afternoon for IAZ004>007-015>017- 023>028-033>039.




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AirportDistAgeWind ktVisSkyWeatherAirDewPtRHinHg
KAMW AMES MUNI,IA 9 sm13 minS 0810 smOvercast81°F72°F74%29.92
KBNW BOONE MUNI,IA 21 sm11 minS 07G1410 sm--79°F75°F89%29.90
KIKV ANKENY RGNL,IA 24 sm11 minS 10G167 smClear81°F72°F74%29.93
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Des Moines, IA,




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