Upadacitinib Combined With Corticosteroids vs Corticosteroid Monotherapy Induction for Inpatients and Outpatients With Acute Severe Ulcerative Colitis

Inclusion Criteria: * Patient ≥ 18 to 75 years of age at the time of consent * Diagnosis of ulcerative colitis (verified by a typical clinical history as well as characteristic appearance on endoscopy and histology) * Meeting the following definition of acute severe ulcerative colitis as defined as having ≥ 6 bowel movements per day with visible blood in the 7 days prior to Day 0 plus at least one of the following: i. Temperature \> 37.8 Celsius(C) per patient report or documented in Electronic Health Record (EHR) in the 7 days prior to Day 0. ii. Pulse ≥ 90 beats per minute (BPM) per patient report or documented in EHR iin the 7 days prior to Day 0 iii. Hemoglobin ≤ 10.5 grams per deciliter (g/dL) in the 7 days prior to Day 0 iv. Erythrocyte sedimentation rate ≥ 30 millimeters per hour (mm/h) in the 7 days prior to Day 0 v. C-reactive protein ≥ 3.0mg/dL in the 7 days prior to Day 0 vi. Fecal calprotectin \>782 Milligrams per kilogram (mg/kg) in the 7 days prior to consent. vii. Oral corticosteroid use for ≥ 7 days in the month prior to consent at a dose equivalent to ≥ 20 milligrams per day (mg/day) * For a person of reproductive/childbearing potential (i.e., presence of intact ovaries and fallopian tubes and are considered premenopausal by standard assessment), a negative lab-based (serum/urine) pregnancy test is required. * For a person of reproductive potential (i.e., presence of intact ovaries and fallopian tubes) and has childbearing potential, intent to use at least one effective method of birth control from study Day 0 through the end of blinding or at least 30 days after the last dose of upadacitinib or week 48, whichever occurs most recently. A person without reproductive or childbearing potential does not require an intent to use effective birth control. * Participants enrolled in the outpatient cohort must be under the care of an outpatient gastroenterologist affiliated with the University of Michigan. * Ability to take oral medication and be willing to adhere to the study intervention regimen. * Participants who are willing and able to comply with scheduled visits, treatment plan, laboratory tests, daily bowel movement symptoms surveys, and other study procedures. * Evidence of a personally signed and dated informed consent document indicating that the participant has been informed of all pertinent aspects of the study. Exclusion Criteria: * On IV corticosteroids for \> 72 hours immediately prior to enrollment continuously (at any institution) which is equivalent to a cumulative dose of 180mg of IV Methylprednisolone in the 3 days prior to enrollment. * Patients with a prior exposure upadacitinib. Previous exposure to other Janus kinase (JAK) inhibitors (e.g., tofacitinib, baricitinib, or filgotinib) are permissible. * History of clinically significant (per investigator's judgment) drug or alcohol abuse within the last 6 months prior to Baseline. (Note: A Urine Drug Screen does not need to be performed). * A history of two or more prior episodes of herpes zoster, or one or more episodes of disseminated herpes zoster. * Patients with ongoing severe active infection (as determined by the study team) per investigator. Patients with active serious infection(s) requiring treatment with intravenous anti-infectives or oral/intramuscular anti-infectives may consider infectious disease clearance. * Active tuberculosis (TB) or untreated latent TB as described in the protocol. * In participants that tested positive for Coronavirus disease 2019 (COVID-19), at least 5 days must have passed between a COVID-19 positive test result and the baseline visit of asymptomatic participants. Participants with mild/moderate COVID-19 infection can be enrolled if fever is resolved without use of antipyretics for 24 hours and other symptoms improved, or if 5 days have passed since the COVID-19 positive test result (whichever comes last). Participants may be rescreened if deemed appropriate by the investigator based upon the participant's health status. * Recent (within past 6 months) cerebrovascular accident, myocardial infarction, coronary stenting, or aorto-coronary bypass surgery. * Known hypersensitivity to the following drugs or constituents (and its excipients): methylprednisolone, prednisone, upadacitinib, or upadacitinib placebo. The following ingredients can be found in upadacitinib and upadacitinib placebo: colloidal silicon dioxide, hypromellose, iron oxide yellow and iron oxide red, magnesium stearate, mannitol, microcrystalline cellulose, polyvinyl alcohol, polyethylene glycol, talc, tartaric acid and titanium dioxide. This includes a known or suspected hypersensitivity to cow's milk for patients expected to be in the inpatient cohort (component of methylprednisolone). * Participants that are currently pregnant or breastfeeding. Participants with a borderline serum pregnancy test at Screening must have absence of clinical suspicion of pregnancy or other pathological causes of borderline results and a serum pregnancy test ≥ 3 days later to document continued lack of a positive result. Participants with a urine pregnancy test at Baseline that is borderline or ambiguous must have a serum pregnancy test performed. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive. * Participants that have received any live vaccine with replicating potential within 30 days prior to the first dose of study drug or are expected to need a live vaccination with any replicating potential during study participation or within 30 days of study completion. * Patients that meet diagnostic criteria for toxic megacolon as determined by the study and treatment team. Patients are expected to have dilation of the transverse colon \> 6 centimeters (cm) or cecum/right colon \> 9cm and three of the following signs of systemic toxicity (Temperature \> 38◦C, Heart rate (HR) \> 120 beats per minute (BPM), white blood cells (WBC) \> 10500/microliter (µL), Hemoglobin